Keystone and vicinity
I don't know much about Keystone proper. Will you have a car? There are plenty of options in Frisco, Dillon, Silverthorne, and Breckenridge, and none would be more than 15 minutes by car (there's a bus, but I'm not sure how good it is).
I usually only grab breakfast, and I like the Butterhorn in Frisco (my favorite), and Sunshine Cafe and Mountain Lyon in Dillon/Silverthorne (both good), plus Blue Moon Bakery for take out breakfast and baked goods (cheap and good). I believe that there are some posts on here about Breck or Summit County in general, so maybe search for those.
When not doing breakfast, I eat/drink at the Dillon Dam Brewery (brewpub food) or Pug Ryan's (also a brewpub, but fancier food, but not necessarily better food). The Dam is a standard brewpub (I don't mean that negatively), while Pug's is a more locals feel place.