Hmm... that might be the sort of thing you'd need a travel agent to arrange, but if you can swing it, that could be fun! Apparently there are some really good used bookstores in Denver...
When I went to TN for guitar camp a few years ago, I came back by way of Nashville and stayed there a day or so. As I recall, it only added twenty or thirty dollars to the price. So...I wonder.
That was different, really, though, because I flew into and out of the same state...just different airports.
But it might be worth looking into.