Gallatin would be quick and easy. If you wanted more laps are simple. If it's got over 3k in the rio it's worth your time for sure.
Shoshone in Cody WY is interesting and not a lot of mile, don't know how good you are on the sticks or how regularly anyone takes a cat down it.
Northgate canyon on the North Platte is excellent with water but you will need to figure out a shuttle and the timing is a little weird because it's super close to the poudre.
Jackson has a couple runs, the Snake I think is more of a float trip with a couple quick shot rapids. Grey's and the Gros Ventre will be more interesting and in. I haven't done either but I get the impression finding someone to run shuttle with isn't normally a problem.
There's more runs in between the two but I think looking into these makes a lot of sense. I'm sure you are already aware but in case you aren't two sites to help your search:
American Whitewater - National Whitewater Inventory
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