The cities in Colorado "near" denver (Littleton, Longmont, Boulder, etc, but nost of all, Denver itself) are almost all surprisingly green and full of trees- when I firt moved here (from the midwest) I was shocked, because it was completely the opposite of my usual experiences (Midwest: trees everywhere but the cities. Colorado: cities full of trees, but the area between the cities is dry and somewhat barren)
If you visit longmont, that is quite far from Denver, but you could incorporate boulder into that drive as well without too much added distance
If you go to Boulder, and you drink alchohol, check out Redstone Meadery, a local mead operation. It isn't in a touristy area, and my wife and I accidentally found it when we went to a bike shop near it, but their mead is delicious, and when we walked in (just to look around) the person inside told us to come on over, and we were presented with a free tasting of most of their lineup! (And it was much better than any mead I have had before) Note, it is slightly pricy- we ended up buying a $20 bottle because we would have felt bad otherwise (free tasting..) but we enjoyed it over the course of a week. (The bottle had a nice built in stopper)
- I am NOT affiliated with them in any way, and have only been there once. However, I felt it was worth it, since mead is delicious!
Don't forget sunscreen! You will be about 3000 feet higher than Las Vegas, and you will probably have at minimum a few hours of bright sun each day
Seeing natural and archaeological sites could be interesting for you. Red rocks etc are beautiful, and the Mesa Cliff dwellings are worth seeing- I remember them from when I saw them as a small child with my parents, and I want to revisit them with my wife as an adult. (They are 7 hours southwest of denver though)