Since this lunch is paid for, it does sound like a fun activity (unless you really dislike your coworkers, then screw it and make a loan payment!). I agree with considering this a gift from the company, rather than a bonus. I'd go the route of buying something you would have any way, and then send the extra $100 to your student loan when you don't have to make that purchase.
My first thought was to buy wine or beer, since these are purchase I would make anyway. $100 would get me at least 6 bottles of wine, enough for a couple months at my usual pace. Or a bunch of singles from the craft beer store - that might be fun to share what I picked, since many have neat artwork or interesting names, and I work with people who also enjoy beer.
But then someone said running shoes, and that's a great idea, too. I'm in the habit of buying my annual pair of running shoes in January, so I'd just push that purchase up to get $100 off my shoes.