In Sonoma the Sonoma Cyclery shop out on Broadway, across from the high school, has a fleet of rental bikes. If you like hills, the locals ride Lovell Valley Road past the Sebastiani Winery, up into the hills and loop around over the Napa County border and back. The road is discontiguous here and there (at ... I think East 7th) where you need to jag left a bit then right on Castle Road for a couple of yards, but just keep going as straight as possible and suddenly the signs read Lovell Valley Road again. Straight until it T's, that is. At that point, turn left, then follow it in a right turn up the hill. It was very pretty up there last Spring when the wildflowers were blooming.

There's also a place somewhere closer to the Plaza that rents out segueways.