Lots of good recommendations so far. I would add to avoid anywhere near Denver, stay further than a 3 hours drive. Reasoning? That is where you will get the most crowding as Denverites escaping the city is very common within that drive range.
Also, do yourself a favor and avoid Estes Park. Estes Park used to be a gem but it has become so touristy, it is much more about money than a CO experience. I will not be going back there ever again.
Southwest CO is great as already mentioned. Crested Butte area is great, same with Buena Vista/Salida area and lots to do in all those areas.