October can see a variety of weather in and around the Rockies and what you will get is totally unpredictable, but a trip to Colorado at this time of year can be rewarding. Rocky mountain NP, Glenwood Springs, Garden of the Gods, Pikes Peak, Cave of the Winds are just a few possibilities out of Denver. At the end of October you would have to accept the fact that you run a risk of seeing ice and snow so it depends on if you want to deal with it. Out of the great places you have already been, I wouldn't discount any of them for a return visit, I'm sure you will find plenty of new beauty spots on a return visit and with only 5 days for an anniversary trip you might enjoy a more relaxed pace to take a more in depth view and relax.