If you don't want it too warm, I'd suggest something in northern CA. Not much rain from late March on, especially in a La Nina year, and the grasses are still green on the hillsides. There's lots of fun riding to be found in Napa and Sonoma Counties. You can head on out to the coast for a change of scenery or cruise into the Central Valley for a day of total flatness (at least once you get there).

If you like hills, don't miss Oakville grade in Napa and Gates Canyon and Mix Canyon between Winters and Vacaville. Oh, and of course you will be required to go to the top of Mt. Diablo and take a picture. Nothing like double-digit grade percentages to get the blood flowing. The good folks on the Northern California sub-forum can give you a lot of route suggestions and may join you for a few.