A couple of thoughts:
For breakfast within walking distance, consider Ina's on Randolph, or the Breakfast Club on Hubbard. Both places also offer lunch fare too. You could also try a cheezborger at the Madison outpost of the "world famous" Billy Goat, just across Ogden from Union Park.
For a later light dinner you could try the bar at One Sixty Blue, one of the city's top casual fine dining spots, or the Tasting Room wine bar, which has excellent food (mostly small plates) as well as a huge selection of wines and a great view of the skyline from upstairs. Both are on Randolph, just across Ogden from the park.
For something more casual, you could walk up Odgen to the corner of Grand and hit up Coalfire Pizza (BYOB, for now) for the city's top (non-Chicago style) pizza, or the Twisted Spoke for a burger on their rooftop deck.
If you search the site you will find threads on many of these options. Call ahead to check hours, as some places will start to close after 10, and many are not open Sunday.
Good luck and enjoy the festival!