The standard tourist ride is over the Golden Gate Bridge and up to one of the ferries for the trip back. Very scenic. You can climb the headlands in Marin or just stay along the water.
For a second ride, I would suggest taking BART to the east bay. Ride up into the hills above Berkeley. Lots of different routes.

"Sean at Lombardi's can hook you up with a quality rental (www.lombardisports.com)

Agree with Theodore - head over the bridge and take a right to cruise into Sausalito (I think that's the town; Im only there as a transient myself) or take a left and do a ride/hill repeats in the Golden Gate recreation area. Must be a better name than that, but that's what the maps calls it.

If you're staying on east coast time, and you get to the bridge before sun-up, all you have to do is hit the buzzer and a guard opens the gate. Money. In the am I think you head over the eastbound (right side) of the bridge as you're leaving town, and in the pm I think its the other side. You're approach to the bridge is the same in either case. Here're some details actually - www.goldengatebridge.org/bikesbridge/bikes.html

You'll dig it"

i rented from blazing saddles recently -- and had a somewhat mixed experience. the bike itself was great, and they did put on look pedals for me.

very long story here, but the issue was that i had a fall, and slightly scratched the lever -- i mean a teeny ding -- and they told me they would have to replace both levers for $200. they refused to process the ride on my credit card until the owner, who was not there, had seen the bike -- i was leaving town, and told them that if they were going to replace the levers, then i wanted the old levers to take with me (since i was basically buying them). eventually they relented, and charged me something like $30 extra to polish the lever.

but the rides out there are awesome. as the others said, ride over the bridge out to marin and you can get in a nice big hill. saucelito is very pretty too. if you have a lot of time, you should head out to tiburon, which has awesome switchbacks. did it by car, and i was white knuckled.