I really just go for the basics: clothes (which include a couple options for days off in town), toiletries (full size rather than travel size like campers generally have), shoes (at least 2 pairs), so on. For my backpack, I’ve got a clipboard, plenty of pens/pencils, a book for breaks, a small notebook for notes or planning, and general basics in terms of personal care that are always good to have on hand (period supplies, sunscreen, chapstick, ibuprofen, pocket knife, bandaids for myself and fellow staff but not campers because their stuff has to come out of the first aid kits, etc). I’m not really one for friendship bracelets or letter writing because when I’m with kids, I like to focus on them and not on my own activities and when I’m on break I like to veg out, check my email if I’m somewhere with reception, shower, or nap. A lot of people bring instruments like guitars or ukuleles. Snacks aren’t a bad idea if there is something you really like that you never get up at camp, like a delicious chocolate stash or something. Other people stock up on things for instant program games and songs, like coloring books, string, bandanas or squishy balls, and so on.
So really, the basics are the same, it’s just having things for your breaks that can really be different or adding in things for entertainment of your kids during downtime if they aren’t prepared with their own things.