Rocky Mountain National Park would make for an excellent day trip from either Fort Collins or Denver. The eastern gateway to RMNP, Estes Park has a number of eateries and a microbrewery. Overnight accommodations are also available. A drive up to and along Trail Ridge Road provides numerous panoramic vistas. Depending on the weather (snow/ice) the road can be closed for the season from early October onward. Even if Trail Ridge Road is closed, the park is still worth a visit. Bear Lake is a popular tourist spot and very scenic. The main tourist onslaught should be over for the season. The United States Air Force Academy is also a good choice for a day trip. Depending on traffic and your location in Denver, it would probably be about a 90 minute drive to the north side of Colorado Springs. If you plan to go on a Saturday, be sure the Cadets do not have a home game as the traffic can be a pain. These are two of the "big" attractions near Denver. If you are an NFL fan, you might try to pick up some Denver Bronco tickets if they are at home. Bronco games tend to be a lot of fun, especially if you wear a team jersey other than the home teams.