Both Frontier and Delta Airlines have round trip airfares travelling to and from Seattle (SEA) and Denver (DEN) for $129, nonstop. This is a basic economy fare, however, on Delta you may have a carry-on bag in addition to a small personal item on board.
Departs SEA: Sept 11, 18, 25-26; Oct 2-3, 9-10, 13, 16-17, 23-24, 27, 30-31;
Departs DEN: Sept 11, 18-19, 25-26; Oct 2-3, 9-10, 16-17, 20, 23-24, 27, 30-31
To book: For the most up-to-date and cheapest airfares, plug in dates to Skyscanner [skyscanner.com]
and select your favorite OTA from there to book it. Skyscanner has multiple choices including our favorites Expedia, Priceline, etc..
For hotels, check out Hotwire's Hot Rates [hotwire.com]