Bring some insect repellant and sun tan location.. Bring all your drinking water and cooking water. It can get very windy even in the canyons. Carefully plot where and when Lewis & Clark camped going up and down river and bring their logbook writings for those days for fun. There are plenty of small hikes that we did. Open canoes - fine, whole trip was just fast water, cold, we never even thought about swimming. The weather can be hot or cold, and you can be wind bound. Bast part was down river of Coal Banks Landing, look it up, there is a small settlement with a very nice general store/outfitter - we rented our canoe there.
Within the first hour a deer jumped in the river and swam accross right in front of us. We visited Indian teepee rock rings and many homestead sites along the river. The people at C B L loaned us a 3 ring binder detailing all the natural and man made things and sites along the river. We went just a bit later than your trip, to camp at the same locations that L & C camped at on the same calendar dates. The river was deserted, we saw, maybe three small groups the whole trip.
After the trip We $ had the people at C B L shuttle us back to C B L, stayed in their VERY interesting collection of original homestead cabins and celebrated with them the 200 year anivarsay of L & C expedition camping at C B L.
Whole trip esp. when viewed from the historical nature of the L & C expedition was great fun.
We also visited some L & C sites on our way to the river, Fort Mandan, Knife River Indian villiage etc.