First, the good news: as long as you don't go to Humboldt, just about any California college town should have nice warm weather. San Francisco gets a little cool and foggy, but in the grand scheme of things, it's not bad.
The bad news: as Rob pointed out, I think you might be disappointed by the public transportation options in many California cities. San Francisco is about the only large city where living car-less is a good option. In LA/San Diego/Sacramento, buses exist but they are not always convenient, and many areas are terrible for walking/cycling. Welcome to American car culture :(
However, many good UC schools exist in smaller towns that are much more livable - small enough that walking/biking are effective for everyday life, and with small but effective bus systems. UC Davis is lovely. UC Santa Barbara also seems quite nice, and I've heard good things about UC Santa Cruz.
San Francisco is probably the most interesting city (certainly the weirdest), but the school selection there is tougher. UCSF doesn't do undergraduate degrees, and Stanford and Berkeley are great schools but very difficult to get into. Maybe there are others there that I don't know about.
Also, check out Pomona. I'm not sure about transportation, but there are 5 colleges there so it should have lots of stuff going on, and Harvey Mudd in particular is a very good school for the sciences (though also difficult to get into).