I've borrowed floggers before, but not everyone is willing to use what isn't theirs. What you should probably focus on is insertables, i.e. vibrators, plugs and gags, because you don't want to use anyone else's. Most of those aren't made of materials that can be sterilized. A first-aid kit is an excellent idea. Also, safer-sex items such as dental dams, condoms and latex gloves are a must. You might want to think about packing an aftercare bag with either a robe or small blanket, drink, snack and soothing lotion for inflamed skin.