Okay, two WACO only places that come to mind and you will be pleased with.
Health Camp for burgers...it is right off the interstate at the traffic circle. Follow the El Chico sign because Health Camp is across the street. Get the Super Health Burger or the Fiesta(it has chili but not positive about the name)Burger. Their malts and shakes are top notch...my family loves the peanut butter.
The other would be Papa Rollo's Pizza on Valley Mills Dr. Their pizzas have a ton of quality cheese but to me, seem a little too salty. Most everyone else loves them.
Elite Cafe/Grill is also on the Circle but hit and miss about quality. There are other WACO only spots but I am not sure they are open on Sunday...World Cafe, Food for THought.
If you arrive and want only a substantial snack there is a GREAT newish yogurt shop on LaSalle out by Baylor. You pay $.45 cents an ounce for your chosen flavor of yogurt and add your toppings. I got yellow cake batter, strawberries and bananas and it was wonderful......however, it may be closed on Sunday as well.