Pack a rain fly, rain coat, waterproof pants, shoes, etc... Weather changes insanely fast and it sucks to be stuck in a downpour! Beautiful campground if Going-to-the-Sun Road is closed halfway through the park.
Best campground for RVs that also allows tents. Just FYI, if you're in a tent then it definitely feels a little less private since the spots are much closer together than at other campgrounds like Jenny Lake.
If you want a great campsite, as most of them are, get there super early!!! Definitely the best campground in Teton but also everyone knows it, so spots fill up by 9am in the summer months. I went in late May one year, a little chilly but still beautiful, and I didn't have any problem getting a campsite. Crowds start in June and go through September.