Ensenada’s is a really good surf and turf Waba Grill has decent teriyaki for a chain Nick the Greek is pretty good Greek food run by some faculty at CSUF Noodle Street and What’s Up Men are both decent ramen places.
There’s a lot around to see within walking distance.
On Chapman/State College, Philz Coffee or Panera Bread for breakfast. Pieology for lunch/dinner. Across the street, Nick the Greek for lunch/dinner or on Nutwood is Bundoo Khan (delicious but spicy). These are all walking distance of the campus. Also, I suggest maybe taking an uber with and exploring downtown Fullerton with your grandma. Lots of options there!