The nice thing about the steamboat town run is that it pretty much parallels the bike path the whole way. So even though the rapids are class 3 the consequences are probably more like a class 2. You can put in up at Walton creek for some flat water boating and take out at fetchers pond if you are happy with that. Fetchers down to hot springs is nice right now with really fun wave trains. One bridge in there that can get pretty tight around 3000. Then the hot springs down to transit center is super fun 3 with really big holes right now that are super fun to just fuckin plow right through- or play around... there's one more bridge in there with about the same clearance as the last one.
I think fetchers to transit center is maybe 4 miles.... if you include the Walton stretch it might add 3 miles, but a lot more time because it's moving a lot slower up there. Don't know for sure about renting a raft, try backdoor sports, but there are usually a lot of people on the river anyway.
It's a good day to be a duck....