Opinions about my trip please!

Hi we are planning a trip to Colorado in August or September and was thinking of not getting too far away from Denver.

Our plan is to arrive to Denver (we are from Argentina), stay for three nights, then rent a car and go to Ogallala (Nebraska) for two nights, then return to Colorado and head to some smaller city in the Mountains. We just want to enjoy the landscape and hang around, not interested in sports.

Which city would you recommend, Aspen or Boulder? Or both? Or some else? Considering we would like to spend a week there.

We would then return to Denver for other three nights before going back to Argentina.

What do you think of this schedule?

Does it get too cold in September or October?

Are the roads that go to Aspen, Boulder or wherever else too mountainous?

Is there another place close to Denver that you would recommend?