SF is great. It's really not that bad for paddling. On a weekend, drive 3 hrs to Truckee, camp, and you're close enough to Yuba, Truckee (boca-florston, though that's more like good III, weak IV), American will be on the way back.
It's not hard to do the same near the Tuloumne, etc
I'm used to CA though. If you have a good car mpg-wise, the sky is the limit. SF is an awesome city, but it's not practical for after-work runs. Weekending trips have great rivers from there imo.