I'm a big fan of the Bay Trail stretch from Emeryville to Richmond Marina and back (or vice versa) - about 20 totally flat miles with one small hill behind the race track - it's a multi-use trail - no cars unless you add little excursions. Much of it is smack on the edge of the bay itself - very pretty. Sometimes can be windy but since it is an up-and-back, you should get the benefit as well as the hit (although isn't it odd how often it seems to be a head wind both ways?).

By looping down and around the Berkeley marina at Cesar Chavez park (I think that's what it's called) you can get another 2.5 miles in. Sometimes it's a little crowded on prime weekend days (the usual hazards of rollerbladers and little kamikaze kids), but not bad first thing in the am or in the evening, and doubly beautiful then. I like it becase there are really only a couple of stop signs - you can actually ride without putting a foot down for a while. And there are a couple of long stretches with good visibility which are fun for sprinting.

And you go past one of the coolest dog parks in the area - Point Isabel - there's nothing to lift the heart like the sight of hundreds of cavorting dogs!

I used to park in the parking lots over behind Chevy's in Emeryville, and more recently, to add a little more mileage, I have been driving out to the end of Point Emery - there's lots of free parking, and leaving from there.