Oh, yes - one of my very favorite subjects. My choices won't appeal to everyone but I love casual food & takeout, for the most part. When I go back there, these are the places I make sure to visit:
Isaac's Deli (several locations) - best sandwiches EVER! http://www.isaacsdeli.com/
Villa Nova (Harrisburg Pike or Columbia Ave) - Tenderloin Tips sandwich with cheese & mushrooms.
Jimmie's Deluxe Grill (behind the Stockyard Inn - Lititz Pike RT 501) - Filet Mignon sandwich with whatever you want on it. Reasonable price & so tasty.
Lancaster Brewing Company (downtown) - Creepin' Dog wings & Buffalo Wings http://www.lancasterbrewing.com/home.html
Alley Kat (downtown) - pizza & Buffalo Fries
Taj Mahal (Rohrerstown Rd - right off RT 30) - Indian food at it's best - http://www.tajlancaster.com/
Tony Wang's (RT 30 East - close to all the hotels & outlets) - excellent Chinese food
Other than those suggestions, you can go to Good 'N Plenty for homestyle food - everyone sits around big tables & passes all the dishes on to the next person. And every chain restaurant that you have ever heard of seems to have a location in Lancaster.
Not a restaurant but might be worth a trip: in Millersville there is a Farm Market: Funk's. They are pricy but they have the best produce. The have baked goods, etc. They usually have the first sweet corn of the year & the last sweet corn. It's well worth the price.
Keep in mind that a lot of places you might want to visit won't be open on Sunday.