The best thing is to grab an armful of USA/Canada brochures from various travel agents and look at what will be the most popular destinations from those, and see which appeal. Colorado is beautiful throughout, there's Aspen for ski runs and just terrific sightseeing, but a lot depends on whether you all want a lazy holiday or a more active one. If you have a special interest in seeing historical sights, though, best to get the travel brochures, because all the pictures will be in there, where they are: Washington, DC, is a super city for sightseeing monuments and museums, Texas would be big fun for rodeos, visiting a ranch, visiting the desert, etc. There are organised tours to all these things via aircon luxury coaches, so no need to feel you're going to be roughing it. In fact, you might prefer not to drive at all, and totally relax, going by Greyhound - very, very comfortable and very reasonable, too.
How about San Francisco? Incredibly friendly and diverse, lots to see and do, visit the local vineyards in the region, buy loads of lovely cheap local fruit, see the Bridge, visit Chinatown, Ghirardelli Wharf, see Coit Tower and the vertiginous Lombard Street, favourite of cop car chases, and so on. Beautiful place.
