As a former camp director, I second the suggestion to check on policies before you tip. If there isn't a policy against it, I agree that a gift card is a good idea, especially something like a Target card, which they can use for all sorts of useful and indulgent things.
Even better than a tip is an email or letter to the camp director telling them how much your child enjoyed the staff member. I used to give end-of-the-summer bonuses to a few staff members who really went above and beyond their job descriptions and parent feedback went a long way. It also affected who got asked back for another summer and what they were paid. So if your child's summer was made even better thanks to a special staff member, let the camp know.