Jeans, slacks, and shorts are the best clothes to wear during the day. Bring a "country-club casual" look with you in the evening.
When it comes to women, the right scarf can make even the most casual outfit look great. It can be worn as a belt, as a wrap on cold nights, or just for fun to dress up an outfit.
Men can wear shorts, khakis, polo shirts, or dress shirts instead of their stuffy suits.
It's always nice to wear a sport coat to dinner. For a week-long trip, one pair of dress shoes that are easy to walk in should be plenty.
Check the weather report for where you're going to be driving to get a good idea of what else you might need.
You should bring warm and cool clothes to layer, a few nice accessories to make things look better, and comfortable shoes.
That way, you won't have to carry any extra stuff with you when you go on the river. Bring sunscreen, a hat, and a water bottle that you can fill up again and again.