Any "Open Space" in Boulder would be great, just make sure to keep the truck running and don't stay too long lest the Boulder Hippies descend upon you (just kidding, of course). I mainly blast them up in Wyoming, since the season in Colorado is a little limited. Check out the regulations at (http://wildlife.state.co.us/NR/rdonl.../smallgame.pdf
) and you will need to get a small game license. There is lots of public land in CO, but you need to get some maps to make sure where they are located. You could ask some ranchers, as many of them are only too happy to let you shoot those things.
I would call the DOW office in the area (from the website above) you will be staying, and ask them about possible places to shoot PDs on local ranches. That has worked for me in the past.