Denver versus Salt Lake City

Plan to add a 3-4 days stopover on our return flight between Vancouver and Ft.Lauderdale - due to award tickets, cannot do western states. It boils down to either Denver or Salt Lake City. We have not been in either one of them.

I wonder if it is feasible to visit Yellowstone from Salt Lake City (need to return to Salt Lake City to fly home) with only 3 - 4 days including driving roundtrip ?

If stop at Denver, we could visit Bouder, and Rocky Mountain Nat'l Park, and what else anyone can suggest ?