What a great trip!
Paddling on the river was easy with barely any current. All the kayaks are two seat ride on tops, so you don't need to be a real experienced paddler.
The hike was moderate easy for experienced hikers - but definitely muddy and far from smooth and the scenery was gorgeous throughout. Our group size was 12 people, and the shuttle van was full so I assume that's the top limit which is nice - you don't want to go with a big crowd.
Our guide Jake was really friendly and helpful, all the while keeping a good eye on the whole group's safety. We stopped several times along the way for interesting explanations of what we were seeing, and at the end Jake was happy to point us to some great local places to eat!
All in all an awesome way to spend the afternoon on Kauai!