gGreat trails in city -- and other options in easy getaway
Denver area trails just keep connecting and connecting, through the city and state parks, and a central spot is the REI near Confluence Park right on the bike trails (the confluence of the Platte River and Cherry Creek) http://www.rei.com/stores/18, they should be able to point you to a good bike rental shop in the downtown area.
If you want to completely get out of Denver at all and your conference is downtown, just walk or take the mall shuttle to Market Street Station and catch the B to Boulder (try for rush hour so you can take the BX express), get off at the Boulder Transit Center, walk a block north to the Pearl Street Mall. There try University Bicycles at 9th and Pearl http://ubikes.com/ for rentals -- not only are you in a very pro-bicycling town, but you're also just a couple blocks north of the Boulder Creek Path, and from there you can ride up into the hills, hook up with trails, and the shop can point you in multiple directions -- road or mountain.
Welcome to Colorado!