I've been going to Colorado off and on for the past 30 years and my favorite thing to do when I'm in the Denver area is visit Red Rocks, perhaps the coolest music venue in the United States. It's a beautiful natural amphitheater up in the hills near Denver. It's been host to many music legends like The Beatles, Bruce Springsteen, and most famously U2. Their show there in 1983 was captured forever in the Under A Blood Red Sky video. I took a bunch of pictures of this legendary place last year.
I've also picked up a few of the many postcards produced through the years. Most of these date from the 60's I believe.
If you're ever in the Denver area, a trip to Red Rocks should definitely be on your to do list. There's no other place like it.