What month?

I like
Rocky Mountain National Park/Estes Park
Mesa Verde/Durango/train
Great Sand Dunes/Pikes Peak/Colorado Springs

Lot of the ski resorts would be nice too.

June or July. I saw something about a train on Trip advisor. Is that a day trip on a train? We would not be able to ski due to the knee issues. Would it still be worth it to go to the ski resort if we cannot ski? Would there be snow? Maybe sleigh rides?

There are at least a couple of well known day train rides in Colorado - one being the Cog Railway up Pikes Peak (close to Colorado Springs in sourthern Colorado)- and another over by Durango - not that far from scenic Telluride in SW Colorado. There might also be something in Rocky Mtn National Park/Estes - north of Boulder - which is north of Denver.

As for Pikes Peak - you do get up to altitude - so be aware if anybody gets light headed or feels a bit ill/strange ("altitude sickness") - and get them some Oxygen - and back down the hill.

And the old saying about ski resorts is: I came here for the skiing but stayed for the summers. They are just beautiful in the summer - and also - sometimes you can take a ski lift up to the top for a spectacular view The "Summit" - with cute little Frisco - or victorian themed Breckinridge is nice - and a couple hours further - is fabled Aspen, very nice in the summer. - wildflowers in the meadows, whatnot.

And while there still may be snow on the peaks. doubt there will be too many sleigh rides going on in the summer. Maybe horse drawn carriages though.

I'd go in July. Peak time. June can still be mud season. The wildflowers are gorgeous late June/ July. Worth the drive. You might enjoy driving from Aspen to Crested Butte through the aspen forests. Actually, I think you can't go wrong with anywhere you choose. Pick which airport gives you the best flights/prices and go from there.

Southern Colorado might provide the best combination of what you seek.

http://www.flickr.com/photos/mksfca/sets/72157622636845987/show/with/4085877115/

If there will be little hiking, the "mud season" mentioned should be of little concern.

May is generally what we view as mud season - heavy melting of snow - but by June it is mostly gone except at higher elevations well above the ski resorts. I would plan a circular route but plan to spend a few days in three or four locations. All of Colorado is beautiful but different areas have different beauty. Ski areas based around a town (Breckenridge, Aspen,Steamboat) have much more to offer than the resort based ski areas like Keystone, Copper Mt, Beaver Creek. etc. Vail, is an exception, as they have worked hard to be a year around area.

If you want to stay put for a week or so, then I would suggest Breckenridge as a center point. Lots of convenient days trips from there, the train at Georgetown, gold mines to tours, some chuckwagon events out of Vail, Independence Pass to Aspen, etc.

You mentioned 'cool places to sleep' and in another post appears you've decided on Estes Park. You should really check out the Stanley Hotel, inspiration for Steven King's The Shining which of course became the movie starring Jack Nicholson.

Have a great trip!