If you want challenging water, I suggest the Skagit, upstream from Marblemount, up to, but not through Shovel Spur rapids. I regularly run the Skagit downstream, and the Chehalis, and Cowlitz. But none of the areas I boat provide the technical challenge you might be seeking. Maybe the Clackamas in OR, I don't know as I've never been on it.
Check the regs. I think the Skagit is closed to motorized boats above Marblemount.