Southwest has round-trip flights from Denver (DEN) to Austin, Detroit, Minneapolis, Seattle, Philadelphia, Chicago, Houston, or Dallas (AUS, DTW, MSP, SEA, PHL, MDW, HOU, DAL) for $$99-$152, NONSTOP. Flights October to January. Departures on Tuesday, Wednesday, and Saturday. Extended weekends available. Origin/Destination reversible.
Philadelphia is the most expensive fare in this group with a price of about $152; the other destinations are cheaper.
Minneapolis and Detroit have a lot of weekend availability. Other destinations are mostly limited to Tue, Wed, Sat.