Yes, six hours will go by quickly. The pedestrian Mall on 16th st that someone else mentioned make for a very nice walk. Also a free circulating bus you can use if you don't want to walk all the way. Union Station is at one end of the mall and the far end is perhaps 45 minute walk if you keep moving. But you will find lots of interesting places to stop along the way.
Do you like craft beer? If so, Denver has a couple dozen microbreweries and perhaps 6 or so of them are an easy walk from Union Station. They all have nice tasting rooms. A dependable one I can highly recommend is Great Divide Brewing on Arapahoe and 22nd. If you want a one-stop place with about 80 different beers on tap, check out the Falling Rock Tap House on Blake St.
May is baseball season, and Coors Field - home of the Colorado Rockies - is a 15 minute walk from Union Station.
And the station itself now has some very nice amenities. If breakfast or lunch time, check out "Snooze". Out-of-this-world breakfast entrees all day. There are some other nice bars and restaurants there also. Their "Terminal Bar" is on the main level inside the station. They have regular seating inside there, but one unique feature is that you can order a beer from a window and then enjoy it in the station lounge area itself. Instead of the old hard wooden "pew" seats they used to have, they now have a selection of couches and comfortable seats to use while waiting for your train. Quite civilized, really!