COLORADO — BEST PLACE TO VISIT IN SUMMER?

So I have this sheet document to use. It expires soon. Where is the most appropriate place to revisit in Colorado this summer (considering the weather, as well as things to do)? Colorado Springs? Boulder? Denver?? Which do we cite as well as what is there to do there? Never been to which state so I have no idea…
I’d usually be there for about 3 to 4 days. So I wouldn’t be means to do the highway outing around the state.