About the Monterey Coastal Bike Trail: Beginning at Castroville, there is a (mostly) off-road, fully paved bicycle trail all the way to Monterey. It begins from a service road next to the 76 station, across the highway from the Burger King. It's a fine ride, and I heartily recommend it. Most of it is rather flat, and with luck the wind will be at your back when traveling southwards. Alas, the wind was not at my back, but I managed.
Along the trail, the first section is past farmland, just east of Highway 1. About a mile past Castroville (I am guessing at distances), the path turn away from the highway, and crosses Nashua Road. It continues along Monte Road, adjacent to the old railroad tracks, past the Dole plant. This section is not off-road, and there will be the occasional truck (or other vehicle) passing by. This section is a mile or two long.
Past the Dole plant, the path crosses Neponset - El Monte Boulevard, and again becomes off-road. This is a very fine section of bike trail, running parallel to El Monte through Marina. Expect to stop, and walk your bike across, the occasional side road.
When you get to the off-road bike path next to Del Monte Boulevard, north of Marina, look to the ground on your immediate right as you go through the low, rolling sand hills. You will see patches of ice plant, remarkable for the color that they add to the otherwise (in summer) brown terrain. In July 2008, the ice plants were particularly attractive, being a mix of green bases shading through orange to bright red tips. It was like a miniature Vermont forest in Autumn, only five inches high. You can see the ice plants on Google satellite images; they are in the area bounded by Del Monte Boulevard and Lapis Road, north of Marina. The satellite does not do them justice.
At the south end of Marina, the bike path crosses under Highway 1. As you rise from beneath the underpass, you reach the crest of the sand dunes that overlook Monterey Bay. You will be able to see the hilltops of the Monterey Peninsula, but not yet the coast.
Along the bike path, looking southwards from north of Marina
Looking from Seaside to Monterey
When you reach Seaside, the trail goes under Highway 1, then immediately turns right and re-crosses 1, so that you are again on the seaward side of 1. If you miss this turn, then you will end up traveling into Seaside, past the commercial areas. As you rise from the underpass to the dune crest, for the first time you will be able to see the bay, with the City of Monterey spread out before you. A little further along, there is beach access, if you wish, but much better access awaits you in Monterey.
UPDATE: In early July, 2008, I discovered that a portion of the bike trail is closed for maintenance. The section is from Tioga Avenue to Canyon del Rey Boulevard, in Sand City (between Seaside and Monterey). Bike traffic must detour onto city streets. Unfortunately, the most likely detour involves a 3/4 mile length of Del Monte Boulevard in a heavily trafficked, bike-unfriendly commercial district. As an alternative detour, you might try (from the north) Tioga Avenue to California Avenue, past the "City Hall" of Sand City. At the end of California Avenue, go left on Contra Costa Street, then right on Olympia Avenue (just before you meet the boulevard). Ride a short distance on Olympia, then right at Roberts Road, which brings you to the back end of a small business and shopping area. Go through the parking lots until you get to Canyon del Rey, then right returns you to the bike path located directly in front of the beach hotel.
Soon, you arrive at Seaside State Beach, which seems to be a favorite place to fly exotic kites. As you pass the entrance to the park, and arrive at the entrance to the resort, look left. The bike trail continues parallel to Highway 1, remaining on the seaward side of the highway. There is an alternate path that goes into the Sand City shopping mall. A little further along, you will again cross under Highway 1, at the other end of the mall. As soon as you go under 1, turn right, along the back side of the stores. Past the stores, a road takes you over 1, and from that point you remain on the seaward side.
Monterey, near Fisherman's Wharf
Soon you reach Monterey, and ride past the Fisherman's Wharf area, through Cannery Row shopping, to the Monterey Bay Aquarium. If you are going to the Youth Hostel, it is inland (uphill) 3 or 4 blocks from the aquarium; at Hawthorne Street (7-11 and Circle-K there), go left one block. The youth hostel is popular with cyclists of all ages; a reservation is recommended, and can be done on-line. Bike lockers are available. UPDATE: In July 2008, you may find it hard to get a reservation at the Monterey Youth Hostel, as apparently some space is being held for possible refugees from coastal fires. Also, if you intended to continue southwards, be aware that until the fire situation is controlled, public transporation does not continue past the south end of Carmel, and the highway itself is blocked at the town of Big Sur.
As you ride through Monterey, you will be on a multi-use path with numerous cyclists, rent-a-cyclists, strollers, toodlers, tourists with cameras, and so forth. Ride slowly, even if you do wear expensive Spandex and carry a hydration pack. Toddlers run around at random, and do not have enough sense to get out of your way. You are not in a hurry.
What's that you say? Where's the restroom? When you reach the tourist area of Monterey, near Fisherman's Wharf, you will see the Monterey Maritime Museum. There are restrooms outside, just off the bike path. They are only open in mid-day, but presumably that's when you will be arriving.
If you intend to take the bus back to Santa Cruz, get it at the Monterey Transit Mall, a fancy name for a pocket park located inland of the downtown area. The bus routes have changed a bit in the past year, so check for current information. More likely than not, you will take a bus to either Marina or Salinas, then transfer to a bus going to Watsonville.
Seventeen Mile Drive is fun to bike. Although 17 Mile Drive starts near downtown Pacific Grove, it is better to do this: From the aquarium, bike along the coast (Sunset Road), past Asilomar. This section is very beautiful. When the road turns inland, you soon arrive at the gated entrance to 17 Mile Drive. All of the Pebble Beach area is a gated, private community, but the entrance fee is charged for cars, not bicycles. Motorcycles are prohibited. Incidentally, they could charge bikes, if they wanted, so it is wise to get current information.
near Asilomar
Past Spanish Bay, follow the signs, and 17 Mile Drive return to the coast. Tip: There is a nice public restroom at Bird Rock. A bit beyond Bird Rock, the bike path turns inland and goes near Spyglass Hill. You can also continue forward along the coast in a more direct line to Pebble Beach, but this section does not have a shoulder for bikes, and you will have to fight traffic. A bit further on, you arrive at the Pebble Beach golf course. A deli and restaurant are convenient (there is also the fancy restaurant, for fancy people, but that's not you or I).
Past Pebble Beach, you remain above sea level and climb slightly. Be sure to take in the scenic views. That is an absolute must-do: The view southward, towards Big Sur, is magnificent. Again, this area is off the official bike path, and there is a lot of traffic. You will need to pull over and let vehicles pass.
When you reach the turn-off to the Carmel gate, it is best to go to Carmel. The first time I did this, I continued uphill to the Highway 1 gate, and it was a long, continuous climb, without a shoulder to the road. The second time, I noticed a sign recommending that bicycles go to Carmel; it was good advice. From Carmel, if you wish to continue cycling, I would recommend retracing back past Pebble Beach. Or, you can take an MST bus from Carmel to Monterey, even on holidays. If you must bike back a different way, there is a path that you can pick up near the Highway 1 gate of 17 Mile Drive (or, ask the locals). Or, you can bike uphill through Carmel via Junipero and Serra roads to Carpenter Street at Highway 1, then go a short distance northwards along 1 to the highway 68 exit (mandatory), then turn off to a bike path that parallels 1 and leads to the east end of Monterey.
I have only described the ocean side of 17 Mile Drive. The complete loop (all seventeen miles of it) has an inland section that leads back through forested residential and "horsey" country. I didn't bike it, primarily because I'm used to that kind of neighborhood. If you haven't seen how the rich people live, you might try it.
On July 5, I rode the bus to Big Sur. During summer season every day, MST runs a bus from Monterey to Nepenthe, in Big Sur (weekends only during off-season). I left the bike in Monterey, had lunch at Nepenthe (just south of the town of Big Sur), and walked around. I do not recommend bicycling along Highway 1 through Big Sur (unless you are an idiot; being an idiot, I did it in 2007, as described here). There is no shoulder, there are numerous hills (and wind), and plenty of traffic looking at the scenery rather than at you. Some areas might be more favorable to bikes, and perhaps some seasons are more favorable, but I wouldn't know. If you wish to bike around, it is better to do it on the Monterey Peninsula.