I was just down in Utah this last week, and got the bug to kayak a desert canyon. I was thinking either Cataract or the Dolores. Cataract would be quite nice. It is mainly a flat float with a big volume canyon in it. It also has quite a long season, especially if you want to avoid the peak flows of May and June.
One option is to do the trip at Easter, however that would put things quite close to peak flows. There are no permits in Apri however. In summer you need a permit , and the temps are quite high. However, it does give one the option of doing a long trip with some good canyoneering.
The Cataract is big volume IV similar to a stout Westwater that gets a bit sustained at high flows. At lower flows it seems like it would be a nice trip for solid boaters.