If you are in Ithaca on a weekend between April and December, you have to go to the Farmer's Market. Honestly, I think if you're there on a Saturday, you will run into every Ithacan stocking up on produce, or just being social and hanging out.
Now, this isn't just a produce market. Yes, you can get freshly grown organic fruits and vegetables, but there are also vendors selling wine from the local wineries (with tastings of course), local craftspeople selling jewelry, pottery, and housewares, bakers offering tasty treats, and people who make their own soaps and beauty products. There are also quite a few purveyors of hot foods, ranging from Indian samosas to burritos, smoothies, Laotian delicacies, Belgian waffles, and sushi. One popular activity is to stay for lunch and eat out on the pier.
Probably the best thing to do besides eating is just people-watching. Everyone will pass through, and Ithaca has some interesting people! You've got your aging hippies, college students, heavily tattooed folks, families, you name it!
The hours are as follows: 9AM-3PM Saturdays from April to October
10AM-3PM Saturdays November and December
10AM-3PM Sundays May through October
4PM-7PM Thursdays June through August
From journal What to do for 5 Hours or 5 Years in Ithaca