You might try programming everything that would interest you for maybe 20 miles around if your interest is to listen to everything you can without a preference. Colorado has pretty good coverage due to the widely varying geography and therefore you will often be surprised how far you can receive sites. Almost everything nearby uses simulcast sites, so you may find that the closest agencies are the most difficult to monitor.
For police, your choices are restricted to a very small number of agencies in Adams, Boulder, and Weld counties, as well as Colorado State Patrol if your interests are traffic stops all day long. For fire, just about everyone other than Denver can be heard. The Denver Fire VocAlarm uses VHF as noted above for station alerting and you can sometimes hear Fire Dispatch on it when the automated voice isn't airing a call. For the rest of public safety, which I assume means EMS, then you have Denver Health that you can monitor.
A reference to check out is the
State of Colorado DTRS Site Map and to try looking at some of the mountain top sites which will give you large coverage areas. The three that readily come to mind are Confier, Lookout, and Thorodin.
