Bring your own refillable water and I would recommend staying away from the snacks sold there. I'm sure that it'll be tempting, but it's expensive and my sister claims that most of it sucks. I'd recommend dressing in cool clothing and carrying some sunscreen with you. Tourists and their kids tend to be a little shitty too, so I'd recommend avoiding them too.

Actually, I'd recommend just avoiding Disneyland all together.

Go to the beach or the Grand Canyon instead.