San Francisco and Marin County have many fine places for recreational cyclists. If you are one of the hard-core types (what I call Spandex Speeders or Velo-Nazis) then you have other sources of information, so this page is addressed to tourists
In San Francisco, it is unwise to cycle on artery roads with heavy traffic. This is true of any city. But there are numerous city streets that don't have too much traffic. If you plan your route, you can get by bike nearly anywhere. However, if you have a particular destination in mind, most Muni buses can carry a couple of bikes, and the major routes have frequent service. Outside of the park areas, the city doesn't have too many places where cycling is an improvement over walking or (if you are in a hurry) bus.
Several places are fine for cycling: The Presidio, Golden Gate Park, and the area including Fort Mason and Crissy Field. But if you were going to Fisherman's Wharf, you will find that it is crowded with vehicles and pedestrians; consider dismounting and walking your bike.
You can rent a bike. One popular route is to get the bike at Fisherman's Wharf or Pier 39, then pedal along the waterside to the Golden Gate Bridge, cross the bridge to Sausalito, then take the ferry back to San Francisco. Unless you cross the bridge at an odd time, you may discover that there is so much foot (and bike) traffic that your progress is slow; if you are not in a hurry, walk your bike across the bridge.
The day I did that, there were some 50 bikes on the ferry back from Sausalito. Most were rentals. I had my own bike, but the cost of rental was very reasonable.
The Presidio area, west of the bridge, has some great views. If you are lucky and the day is clear, be sure to visit the "north beach" area for the spectacular view across the entrance of the bay. Also have a look at the spectacularly expensive mansions. I believe that if you seek to buy one, the price is measured in oil wells, not dollars. It is more fashionable to buy a block of them by trading in a moderately sized third-world country. They don't like like much from the outside; many are original construction that has been gutted and refurbished inside.
Golden Gate Park is fine for cycling. Keep in mind that bikes are not permitted on many of the pedestrian paths. You will have to use ordinary roads, or walk your bike. Some days, the streets are closed to vehicles; I am not sure if the cycling is better on those days.
Western Marin County is more rural, and is well-suited to the tougher recreational cyclist. If you rarely cycle, don't do it: most of the roads have no separate bike path, and they have a lot of steep grades and turns. Bus service, via the West Marin Stagecoach service, has very limited capacity for carrying bikes. So, if you bike over a hill into the next valley and hope to take the bus back out, you may be out of luck. That didn't happen to me, but it did happen to some heavily-packed tourist who couldn't get a ride because other cyclists (such as myself) were already on the bus.
When I went there, I took the Golden Gate Transporation bus to Marin City, then changed to the stagecoach. I had a "plan B" in case I couldn't get the buses, but fortunately I could load my bike. When I got off at Pantoll Station, located most of the way up Mount Tamalpais, it was hot. Hot, I say. In fact, it was about 25°F above the seasonal average high for that day. Show 360 degree panorama from Bolinas Ridge.
I biked awhile along the ridge, then decided it might be a smarter idea to descend to Route 1 rather than take Bolinas Ridge fire road. But it wasn't a smarter idea. Had I stayed on the ridge, I would have traversed more or less level ground until I descended near Olema. As it was, I descended near Bolinas, then had to climb some 400' between there and Olema. I did mention that it was hot, didn't I?
Olema has a convenience store and a restaurant. I doused myself with a bottle of cold water form the store, then continued to my destination, the hostel in Point Reyes National Seashore. I knew that I would have to climb a ridge to get there. But the usual weather is cool and foggy, suitable for a long climb. That day, it was so hot that I had to dismount and push the bike up. Twice, passing cars stopped and asked if I was OK. Yes, I had merely reached the "yellow line" of my bio-tachometer. I would make it. When I got to the hostel, I was so heated that I grabbed a couple of ice packs from the freezer (fortunately, they had them) and put them over my head and neck. The next day, I met a cyclist who was much younger and dumber than I. He had pushed it further, and was quite sick. Moral: Don't do what you can't do, especially you recreational cyclists.
Somewhere along the way, I broke a spoke. Fortunately, on weekends (only), there is a portable bike shop at the north end of Pt. Reyes Station. They had the right spoke.
I returned via stagecoach. Numerous cyclists were suffering along the roads. Some day, I'll go back and bike through the magnificent hills and valleys. Given that summer weather seems to alternate between cool fog and blazing heat, perhaps the right time is between rainstorms in winter. Judging from other web sites, the locals are always biking the area, especially along the ridge. Since I live in an area that is also good for cycling, I can wait for better weather the next time I go to Marin.