Anywhere that isn't running too muddy, high, and fast should be great, as long as you have the right tackle.
I was in Steamboat Springs last October. The Yampa is a gorgeous river. Great fly shop, right there in town.
I got to Colorado in late September, about a week after Boulder Creek, the Big Thompson, the St. Vrain, and the RMNP rivers and streams went over the banks, unfortunately. I did get a look a Boulder Creek, a great little stream right in the middle of town, with a running path paralleling it for miles. Also, another great fly shop, right in the middle of town. Locals cap on Boulder Creek because the fish size tops out at about 13". When I was there, the flood waters had receded, but it was still running full bore. Easy to see what a gorgeous little stream it is, though.
Also in Golden, another great fly shop, right in the middle of town. The guy there was telling me about Euro nymphing Clear Creek with a 25' leader in the lower reaches, where it's really big water. Clear Creek doesn't really turn into a "creek" until you get up near the headwaters, something like 40 miles upstream.
Two things to keep in mind about Colorado- 1) it's often very windy; 2) the water is often big, wide, fast flowing, and deep. I'd bring more than one rod, but unless you plan to stick to headwaters, small tribs, and spring creeks, take a 9' 6-8 weight wind-cutter as the first choice. There's plenty of technical dry fly fishing and tiny-fly nymphing for big fish, but even there, you'd be better off using a 6 weight than a 4 weight if you're on the main rivers and tailwaters. There were times when the wind made it very difficult for me, even using a 7 weight with a sink-tip. Also, because the flows are so fast, using a lot of weight is common when nymphing or streamer fishing. Chuck and duck.
oh yeah, 3) lots of good water is posted and fenced off. The fly shops can help you out with finding public water. Boulder Creek is all public. The Yampa mileage in the town of Steamboat Springs is all public. And it's terrific. Not the dregs at all.
Plenty of info on-line about Colorado, too. So many great rivers there. I'd map out the road route, find the rivers that run nearby, and do a search on each one along with the keywords [fly fishing report 2014]