There's riding and snowshoeing without leaving Keystone. Besides getting a feel for what more advanced adventures could be like, you'll get some pictures you'd never get on the ski slopes (see the gallery in the photography section).
Beginners will enjoy riding around the local area, covering more ground than you could walking.
The easiest trails should be comfortable for anyone who can ride a track 20" wide. I snowshoe and walk the local trails everyday. Once we start getting regular snows, daily use keeps them very smooth. The new tires are as grabby as a regular bike in dirt. It's like riding a bike!
If you leave the single track, it's interesting - similar to crashing in powder on skis, except you have absolutely no chance of losing the bike. And this will amuse your dog.