The flattest, boringest ride in SF is doing loops at the Polo Field in GG Park. The track is approximately .65 miles long and if you go between 5PM and 7PM, particularly Tuesdays or Thursdays, there will likely be other cyclists there.

If that's too boring, try this:
1. Start at the eastern end of GGPark, intersection of JFK Drive and Stanyan.
2. Head west toward the ocean on JFK.
3. At the end of park, turn left onto The Great Highway.
4. When you reach the zoo, turn left on Sloat Blvd.
5. Turn right onto Skyline.
6. Turn left onto Lake Merced. (This will involve crossing a lane, or maybe two lanes, of potentially fast moving traffic to get into the left-turn lane.)
7. There's a bike path that goes around Lake Merced. At first you'll be on the road riding alongside it. Eventually, you'll see a curb cutout where you can easily get onto the path. You don't actually have to get onto the path, but traffic on Lake Merced gets a little heavy/fast at this point. So I'd recommend getting on the path, and staying on it until you reach the intersection with John Muir Drive.)
8. Turn right onto John Muir. (This is where I get off the path and start riding the road again.)
9. Turn right onto Skyline.
10. As you head north and see stop signs, merge left into one of the stop sign lanes. (The right-most lane continues north with no stop signs. Watch for fast-moving traffic as you merge into one of the two other lanes.)
11. Turn left onto the Great Highway at the stop sign. You'll be going up a little hill....the most climbing of this route.
12. Take the Great Highway back to the park. Turn right onto Lincoln.
13. Almost immediately, make a left at the stop sign into the park. You'll be on Martin Luther King.
14. Turn left onto Middle Drive East.
15. Middle Drive East intersects with JFK Drive, and you're basically back where you started.

This ends up being like a 16 - 18 mile loop.