I would check out the Forney, and there is an incredible air museum, Wings Over the Rockies, that will take about four hours to see. Make sure to try your hand on the Wright Flyer simulator while you are there.

I can recommend The Academy Hotel in Colorado Springs if you get down that far. Check out the Garden of the Gods and if there is not something special going on there then visit The Bishop's Castle.

Just north of the Forney, just north of I-70, is an old cemetery where a lot of Denver's founding fathers are buried. Lots of weird 'Monuments' there if you are into that sort of thing.

And if you can catch a concert at Red Rocks then by all means do so. If not then just go during the day and check the place out. It is amazing.