There are several good airfare deals today Denver to Europe for departures in April and May 2017, like Paris for $385 round trip. Amsterdam, Brussels and Luxembourg are priced in low $400s.
United Airlines Denver DEN– Chicago ORD – Montreal YUL – Paris CDG – Washington IAD – Denver DEN
$384.76 round trip TravelMerry.com ticket
I found these low fare dates using Google Flights, then searched Momondo.com for this fare.
KLM codeshare for Delta operated flights.
Denver DEN – Atlanta ATL – Frankfurt FRA – Detroit DTW – Denver DEN
Denver DEN – Atlanta ATL – Brussels BRU – New York JFK – Denver DEN