Good class III on the South Fork American. Try the Chili Bar and Gorge sections. More play on Chili Bar, longer day on Gorge. Also, Chaimberlain Falls on North Fork American (III-IV, maybe harder with high water) is supposedly great. I haven't run it myself (haven't had the opportunity), but have heard great things from local paddlers. Seems to only run from rain runoff though, so it may be luck of the draw.
Last year we ran the Trinity and Cal Salmon in March. Great fun. Try Pigion Point on the Trinity, and Wooly Creek to confluence with Klamath on the Cal Salmon.
Really, there are a ton of great runs in Cali. Check out boof.com for a forum similar to this, and creekin.net for a good online river guide. Have fun!
"A witty saying proves nothing."
- Voltaire (1694-1778)