Excellent question! We did book a hotel block at the Ridgeline in Estes Park, but there are some beautiful AirBnB's in the area. For the adventurous, there are also some campsites!
We would prefer semi-formal attire - it is a special day and a celebration! But be sure to remember to be sensible. We're getting married in Upper Beaver Meadows of Rocky Mountain National Park, and there's no telling the terrain we'll encounter (mud and high heels just don't mix!).
From our research, there's an average high of 56 degrees and it's described as 'clear, crisp air, blue skies, and generally dry weather'. This to us sounds like the perfect fall, just remember to bring a sweater!