Hi
Given that you get to Denver by 5PM I am presuming you are there mid morning to mid afternoon. If it's a hearty breakfast you need, take a cab to the Park Cafe on 20th St. Fantastic home made sausages etc. You could then head down to the previous poster's recommendation and go the the 220 Coffee Collective..oustanding espresso from people who are passionate about making great coffee. 20th St is a little "rough n ready" but you'll be fine and those 2 places are well worth visiting. The neighbourhood is on the up.
If it's lunch you are after, I would either head Downtown to Souleio on 3rd Ave (excellent deli, interesting lunches) or go over the bridge to Broadway and go to Christie's Il Secondo (802C Broadway) and enjoy some of their fantastic wood-fired pizza (Tues-Sat from 11.30). They also pour a very good shot of espresso.
If you don't mind drinking during the day, and love interesting beer, you could also go downtown to the Senator Hotel and visit Winston's Bar for lunch..good pub food and a fantastic menu of hundreds of bottled beers from around the world and a good list of beers on tap.
Hope this helps. Enjoy your visit
YXEG