Not true! Denver is a great city with lots of things to do!
Denver has tons of delicious restaurants of all types, ethnic and American. I would suggest these two websites to find great reviews for restaurants, bars, live music, sights, etc. while you're here. I would suggest just staying right downtown in a hotel if you can afford it (with the exchange rate these days I'm sure you'll be fine). Then just cab it/walk all over from there.
http://denver.citysearch.com/
http://www.westword.com/
Aspen is a beautiful town, and even more so this time of year with the change of seasons. Lots of shops and restaurants, hiking, biking, and other outdoor activities. Go to the nearest visitor's center for more info, but it's small enough that you won't feel like you're missing out on something by not doing your research first.
Never been to Moab.
Gunnison- never really stayed there, but have been camping etc. nearby. Lots of hiking/hot springs nearby/rafting/outdoorsy stuff (I certainly hope you are into the outdoors to be hitting all of these locations).
http://www.visitgunnison.com/index.cfm
If you're bored in Gunnison, take a trip up to Crested Butte and stay the night. This town is absolutely gorgeous and there are lots of shops, restaurants, and bars. Just a really cool mountain town and the drive there is beautiful. Of course, the Great Sand Dunes are totally amazing and along your path so you should definitely try and check that out!
Also, you've got to see the Collegiate Peaks mountain range, which is north of Salida (google maps it). Buena Vista is the town near there with a nice hot springs and rafting galore.
Haven't spent much time in Salida, but 1 night is probably enough to see it all and move on (in fact, if you can skip it and see Crested Butte and Buena Vista, do it).
Lastly, I have only been through the Springs, so I can't give you too much advice about it. Fairly good size city though, so I'm sure you'll find lots to do. Check out Cave of the Winds (then tell me if it's cool), Pike's Peak (one of the more popular 14-ers in Colorado), and the Royal Gorge (a must-see) which is located in Canon City (between Salida and CO Springs).
If you have done any research, I'm sure you realize how many national parks and forests there are. You picked a perfect time to visit CO. Just don't forget to bring a coat (nights will be pretty chilly here by then, esp up in the mountains if you make it up to them).
I hope I have helped you out a little bit!
Have a great trip!