That Denver needs a little help, but not much. I wouldn't go crazy topping it with expensive stuff.
A lot of people use
AB Lifestyles We use an inexpensive
5-zonetopper. We ordered a full queen and trimmed it.
It can be covered by any type of mattress pad cover. For chilly nights , we covered ours with a heated mattress pad that has worked very well for us for the past 8 years.