I'd pick a couple of things that would be interesting at different times. It's a pretty small space though. Whatever you decide, there's ALWAYS room for some bulbs. I'm not sure how rain lilies (my favorite) would do in CO, but you guys can grow some stuff that we can't down here with warmer winters. Pack them in there and you'll have something in bloom all season.
That could also be a pretty great spot for a terrestrial orchid.