Colorado's Rocky Mountain National Park is always gorgeous - but in my opinion, it's especially beautiful in the fall. By mid-September, the tall aspen trees in the park turn a brilliant gold, standing out against the high mountain peaks. Walking around Bear Lake is one of my favorite ways to take in the fall foliage. Just keep in mind that if you're visiting the park through mid-October, timed-entry reservations are required for certain parts of the day.