The South Fork of the American River from Chili Bar down through Coloma to Folsom Lake is the most popular whitewater run in California, and a good introduction to commercial rafting for anyone who has not done it before. The water is Class II and III: bouncy, fun, with named rapids like Troublemaker and Satan's Cesspool, but not punishing.
A dozen or so outfitters run trips out of Coloma between April and October. Half-day trips cover the Coloma–Lotus stretch (~12 km) and full-day trips add the more technical "Gorge" section below. River flow is dam-controlled, so the season is more reliable than on free-flowing California rivers. Book ahead on summer weekends.