Juanita's in Century Square (on University across from the Zachary Lot/ Polo Field area), is nice for atmosphere. It's a newer restaurant, might be a bit of a wait depending on timing with students back. But the atmosphere is nice. The green area in front is a nice place to hang out while you wait for a table, I think they have live music on Sunday nights. Parking on street is free on Sundays, or the garage isn't far away to park and walk. Porter's Steakhouse is in this area too, it's a nice steakhouse option.
There are other stores and shops to walk in visit in the vicinity, not sure if closing times on Sunday.
Napa Flats on Harvey, patio seating would be nice with the weather cooling a little.