Tim Horton's @ Ottawa Macdonald-Cartier International Airport

Coffee, sandwiches, doughnuts, etc. Tim Horton's is sort-of like Dunkin Donuts in the U.S., but much, much more popular.

To add to Tony's comment, there's also a Tim Horton's (like Dunkin Donuts in the U.S.) about a 5-minute walk from the Ottawa Flying Club, so it's easy to walk over and grab a coffee or a bagel.