DEN looks close to Denver until you time the full trip. The airport is far from downtown, so the A Line is useful, but it still needs a serious buffer.
What to do with bags
DEN has offered remote bag drop for certain domestic departing flights, but that is not the same as layover luggage storage. Confirm airline eligibility and cutoff times before relying on it.
Leaving the airport
Union Station is the cleanest city target by rail. A shorter layover is usually better spent at the airport hotel, outdoor plaza, or a close food reset.
Getting back through security
Add time for train frequency, the walk back from transit, security, and the underground train to your concourse. DEN rewards early returns.