My wife and I live in Denver and own a property in Keystone. Needless to say we're in Keystone a a lot. If you're in the mood for fine cuisine in Keystone we've visited them all (Alpenglow Stube, The Ranch, Der Fondue, etc) My favorite is the Ski Tip Lodge. Everything about this place is quaint. It's small cabin feel and it's home-style decor make you feel like you're at grandma's house.
The food is world-class. They try to keep local game and wildlife on the menu at all times. They have other options, but if you have the taste for some local trout or duck you'd best try that. I've never had a bad meal.
Ski Tip Location: It's also very easy to get to from River Run. This is a plus over the Alpenglow and Der Fondue since you have to take two unheated gondolas to get there (30 minute journey). It's exciting to ride the gondola at night for the first time, but after about 5 minutes you'll wish you were back in your heated car.