hat with a tie
If you bring a knife use a folding locking blade type. Knives hanging on your belt are a danger to those trying to rescue you and if you lose it.
Cooler for hot days
Gas money for the drivers.
Clothing:Bring a shirt, shorts or pants to cover up or protect you from a hot sunny tube or friction on your arms from the tube.
Your trip leader goes way out and wears pajamas and a hat. The bottoms don't hold up well on the rock if you are the rock sliding type. No sun for me. 50% cotton 50% plastic tight weave is the best. Don't take all cotton on a cold day nor your winter stuff on a hot day. Soaking winter underwear on a hot day does not cool you off.
Men will find that speedo type swimwear will avoid that ever damp feeling under your pants AND let you take a lunch break in your swim suit. While jeans can work well canoeing for rock protection during cool days they will not work at all for tubing. Jean shorts do work well.
You can also use towels on the tube rather than pants. Most people wear a quick drying swim suit as a basic item. Foot wear is a must.
Extra items can be tied on or left in the canoe. Do not have loose ropes hanging out.
See Style on tube types and think about a spare.