Fly nonstop from New York JFK to Denver, Colorado for $155 round-trip on JetBlue in September.
This fare is only available on a few weeks starting from September 10, through the end of the month. Travel is available on any day except Sundays. If you can't find a date pair that works for you there are other one-way flights available for only $99, so you can combine those for a $177 fare.
There are a few days available for this fare price in the spring, but you will need to be flexible with dates.
Searching on the JetBlue website, I could find seats departing JFK Saturday, September 22, and returning from DEN on Saturday, September 29, 2018.
Need a hotel in the Mile High City? Here's what's available in Denver for the listed dates on Hotelwatchdog.