Insulating layer. Bring along a synthetic pile jacket or wool sweater just in case the weather turns out to be especially chilly. You can always leave it in the car or tie it around your waist to wear at lunch and rest breaks. Nice to have!
Warm hat. You've probably heard it before, but more than half of your body heat escapes through your noggin, so bring a fleece or wool hat. Since some outfitters require helmets be worn, the less bulky the hat the better.
Socks. Wool or fleece socks are essential. Again, cotton socks are a killer.
Footwear. If your outfitter provides neoprene booties, you're set. If not, then wear sport sandals or running shoes with neoprene or Gore-Tex socks pulled on over your regular socks.
Snack. Find out from the outfitter if lunch or a snack is part of the price. Even if you'll be fed, it's a good idea to pack an energy bar or other snack packed in a plastic bag and stowed in a pocket for a quick boost of energy on the river.
Gloves. If you happen to own neoprene paddling gloves, bring 'em. If not, a pair of fleece or polypropylene gloves will do the trick of keeping hands warm and helping stave off blisters. Just be sure any glove you bring has some "grip" in the palm and finger areas.
Glasses strap. If you wear prescription eyeglasses or sunglasses, then by all means bring a retaining strap.
Baseball caps. Excellent sun protection on sunny days.
WHAT NOT TO BRING:
T-shirts. Unless it's hot, leave the cotton shirt behind because once it's wet it continues to chill the body through evaporation much the same way a refrigerator operates.
Jewelry. Leave it at home or you'll lose it forever.
Wallets. Best to leave it safely stowed in your car, or tuck ID into a zipper-lock bag and stuff it in a zippered pocket.
Car keys. Self-explanatory? Thought so.