Extra water bottles, snacks (something that will actually give you energy, like granola/protein bars), sunscreen, chapstick, baby wipes (these are a must for me!!), small first aid kit, extra socks (sounds weird but there’s nothing worse than having to spend the day in wet socks because you got water in them or you’re sweaty haha!), deodorant.
That was a very random list but I feel like those are the things that I always find useful in the summer! Hope it helps!!!