Agree with the others, about US 36, particularly across Missouri and Kansas. It's a pleasant drive, not difficult at all. While Kansas is not exactly the most exciting state to drive through, there are some incredible vistas to enjoy. For a leg-stretching session, you can stop at the "Geographical Center of the US" near Lebanon KS, just north off of US 36. Old abandoned motel there (someone had a vision at some point in time) and a quaint little park with signs marking the location. Good place to stop and get out for a bit.
When you get to St Francis KS, you could turn north on KS 27 and take that up to US 34, then take US 34 into Colorado, all the way through to Greeley and on to Loveland.
When you get to Loveland, you won't be able to continue on US 34 (nor would you want to) as Trail Ridge Rd through RMNP won't be open until late May at the earliest. You can take US 34 (E And W Eisenhower Blvd) west to Wilson Ave, and take that north into the west side of Fort Collins, where it becomes Taft Hill Rd.
You'll bypass a lot of the traffic in and around Fort Collins, and Taft Hill Rd will take you to the US 287 bypass around Laporte, which in turn will take you to CO 14 which runs west through Poudre Canyon, up and over Cameron Pass, and drops down in to North Park, the area around Gould and Walden. There's tremendous opportunities in Poudre Canyon (many USFS campgrounds open around May 15).
Continue on CO 14 through North Park and you'll meet up with US 40 at Muddy Pass and then take US 40 over Rabbit Ears Pass and then drop down into Steamboat.
None of the passes on this route are overly difficult. Cameron tops out at about 10,400 and is a somewhat gradual climb on the east side. Muddy and Rabbit Ears aren't difficult at all coming from CO 14; the drop down US 40 from Rabbit Ears to Steamboat is somewhat long, but it's a good road.