Imyers pretty much hit the nail on the head. By August, especially this year it's going to be done. Even the narrows will probably be out by then. Normal water years we can boat through August but basically, anything after 8/1 is a bonus.
Lots of river to be explored, especially at the top of the canyon if you are a class V boater. Again, mostly earlier in the season.
As for rafts, 14' works just fine, smaller boats extend the season, normal peak is first week of June, so after that it's all downhill, 10.5' rafts are the way to go if you are buying one.
Work on the new play park in town is supposed to start this fall and be ready by the 2019 season, but don't hold your breath, it's been years in the making.
P the K