MWP, I really think you should consider renting a car. I have been to the casinos in Colorado, and they are okay. The games are okay. But the drive is breathtaking. Public transport won't allow you to get off the beaten path. Within an hour you can see Rocky Mountain National Park, or go the other direction to Pikes Peak and the Garden of the Gods, or the Airforce Academy.

As others have said, don't rent at the airport. Maybe take the train or the bus to the first casino, and rent a car from there. But as much as you talk about the scenery in your trip reports, I think you'll be disappointed if you don't rent a car. You will miss a lot.