Not quite the altitude you're looking for, but we enjoyed a week in San Pedro de Atacama, Chile (8,000') in March and could easily have stayed much longer without running out of things to do. Several days we were up around 14,000' but we didn't sleep there. There were lots of places to look up at from that altitude, too. It was T-shirts and shorts weather in town, but chillier at higher altitudes. (Plus, you want to get to the geysers before sunrise because the sun kills off most of the steam, and the stars tend to look their best after dark.)