You probably don't need AT bindings and boots. There is some decent lift-accessed side country in most resorts that you can take the lift up, go out a gate and boot pack to the goods. This is my preferred method because I get to ski real alpine equipment which is still better than AT gear for downhill skiing. You can always go get started at the top of Loveland Pass. It's high and wind swept, so there is a lot of decent terrain that you can walk in your boots to.
However, don't think about doing that without a beacon, shovel and probe and more importantly, how to use them. If you are in Colorado, you should definitely take a avy course and get out of bounds. It's great exercise and you can find better terrain than in bounds.
Another good place is Berthoud Pass. The more you go to sidecountry areas, the more you will want to get further and higher, at which point you will probably want to get AT bindings and skins.