Our favorite winery is Hermann Wiemer, amazing award-winning German style wines. Bully Hill is lots of fun, they have an interesting wine museum of sorts and very good restaurant w/ covered deck overlooking the lake. Wine slushies are excellent.
Dr. Frank's is very popular. So many wineries - also many craft breweries. Tabora has a deli type menu with deck. Any of them, I'd double check before heading out. Covid is unfortunately ramping up again and some places have altered their hours/policies. Hunt Country in Branchport is doing curbside tastings, lol.
Ports Cafe (W Lake Rd Geneva, so Seneca Lake) is excellent. Stonecutter's or Edgar's @ Belhurst Castle. Stonecutter's is more casual and has outdoor seating. Water St. Cafe in Geneva for breakfast or lunch, but little outdoor seating (Quilty Pleasures is small, but just catty corner across the street!). Uncle Joe's in Geneva for Italian - a couple of tables on porch. Or grab pizza @ Cams in Geneva or PY - thin crust and delicious. Only Edgars would be dressy.
Watkins Glen - south end of Seneca Lake has the marina (very casual) and Seneca Harbor. The True Love, a schooner that was in a couple of movies is there and great fun to go on. It's a couple of hours, I think, so your whole day isn't committed, but something different.
Food on Keuka - Top of the Lake, Union Block (no outdoor seating?), down the east side is Olney Place for great sandwiches and cinnamon rolls. Hammondsport - Union Block in the village or Waterfront on the lake (west side, almost at end of lake) or Lakeside.
If you plan to cook/grill @ your rental, go to Morgan's in PY for fabulous CAB, the only place I buy beef. The "regular" grocery in PY is a very small Tops. In Geneva, you will have Wegmans and a WM Superstore, but there are a lot of produce stands everywhere (not open on Sundays as they are mostly Mennonite run). Depending on when you come in, you could go to the Windmill for fresh produce and a stroll about as well - although it's more like a flea market than craft market as it used to be.
Yeah - probably more than you wanted, but you can look at them online and see what appeals. Have a great time!