Hi Les! Great choice. I am a Colorado girl, and only recently moved away. It is a wonderful state and you will have a great time. Boulder is a great place to stay if you want both outdoors and easy access to city stuff and the airport. There are lots of trails you can hike, Pearl street can be fun for dinner and there are usually free events like concerts and festivals on the weekends. If you have a car, the Garden of the Gods down in Colorado Springs is also a nice, free place to hike or have a picnic.
Two things to keep in mind 1) DRINK LOTS OF WATER - it can get hot and you'll be at a high altitude (even if you stay around Denver) so drink a lot more water than you think you might need. and 2) Be good to yourself - it is a high altitude, especially if you go into the mountains, and if you've never been up around 12,000 to 14,000 feet you can feel sick. So take your time, don't try to do too much hiking or climbing all at once and drink lots of water.
Dinosaur National Monument.
The view from Lookout Mountain (over Denver).
Garden of the Gods. Pikes Peak, if you are a brave driver. Eat bison/buffalo meat.
Colorado Gators Reptile Park.
Great Sand Dunes National Park -- just like being at Dune du Pilat, near Arachon, in France -- except there's no ocean!
Be aware that Colorado is about 1-1/3 as large as all of Great Britain, so the distances may be surprising -- but then, so are the elevations and the sights!