You have options.
1. Conveniently located in Terminal 2 an AA day pass lounge as dhuey described.
2. Your boarding pass will also get you into other terminals if visiting other the lounges or restaurants, yoga room, or artwork. (Note: UA shut down day passes for people without an AA boarding pass) Here is a list of restaurants. some are quiet good.
Samovar Tea.
https://www.flysfo.com/shop-dine-relax/dining.
3. Take some transport away from the airport as Oakshadow mentioned. a) BART to the city is an adventure in time management and for sights with about a $19 roundtrip fare. b. Short Lyft/Uber/? to South SanFrancisco Main Street for Cafe Bunn Mi or Dumpling Empire. or to Burlingame near Burlingame Avenue
4. Free Airport Hotel Shuttle to a hotel restaurant. (my last choice)