What should I wear to a summer visit day at Kenyon? It’s supposed to be 90 degrees. I do not have an interview. There are several different information sessions and a tour.
Shorts, t-shirt, comfy shoes for walking. No one (and I mean NO ONE) will particularly take notice of you unless you strip naked and dance across the lawn. Don’t wear a shirt with the name of another college, though. 
Where anything you like. Unless you schedule an interview. If you are making the effort to get there, you should, quite frankly.
I don’t necessarily agree on the interview. Not every kid interviews well, and it can hurt more than it helps if that is the case.
Agree, that there is no need to dress up – but I’d still keep it neat and clean. You may bump into admissions officers and/or future classmates and don’t want to stand out in a bad way. Nothing at all suggestive (if female), no ripped clothing, no shirts with other college names as @intparent noted above etc. Shorts (not super short), comfy shoes/sneakers, and a neat shirt should do the trick. If female and would prefer to wear a sundress in the heat that is fine too (ir it is not too short, not suggestive).