One of my absolute favorites in CU, Courier Cafe is the best place to relax and get some delicious American food. Old fashioned soda fountain favorites like phosphates and malt shakes make this place unique, but nearly everything is good. Sandwiches are on fresh bread, and burgers are thick and hearty. There are a lot of vegetarian options- my favorite is the pesto grilled swiss on sourdough. I also love the ham melt on a fresh croissant. For dessert I recommend sharing a headliner, a monstrously delicious sundae drenched in hot fudge.
For the quality of food you get, this place is actually a bargain. I'd prefer it over most of CU's "fine dining" establishments any day. Service can be slightly slow, but overall this is one of my favorite places to enjoy after a difficult test or a arduous paper-writing spree.