Of course you'll need a bathing suit. Make sure that it's comfortable and fits well. Don't wear something that has metal buckles, they're not allowed on many waterslides. If girls are riding down steep slides they might want to wear a one-piece to avoid potential embarrassment. Anything that is tied may easily come undone so it's probably a good idea to avoid ties. You'll also want something for your feet. Water socks or swim shoes are a good choice because you don't need to take them off for most waterslides. Flip flops probably aren't allowed on the slide, but you can leave them at the bottom; most parks have a designated area near the exit of the slide. Bring a hat, but leave that at the bottom, too. If you bring sunglasses use a strap so they won't get lost. Don't forget a towel. A large beach towel also makes a good bathing suit cover.
Nothing will ruin your day faster than a sunburn. Make sure that your sunscreen is waterproof and follow the directions for reapplication on the package. If possible, apply it before you leave for the park and reapply it throughout the day. You might want to bring lip balm with sun protection as well. Sunscreen is always important --you can get sunburned even on a cloudy day, and there can be major consequences later in life. It's not worth the risk, so wear your sunscreen.
Leave the digital camera at home. As an alternative, consider purchasing a waterproof disposable camera to bring with you. You can end up with some great pictures, but you won't have to worry about water damage. If it gets lost or stolen, you're only out a few dollars.
With the exception of an inexpensive waterproof watch, leave your jewelry at home. Don't bring air filled water toys or water wings unless you know that they're allowed at your destination. Forget the snorkel mask, it's probably not allowed and you'll just be stuck carrying it. Don't bring more than a few dollars unless you're able to rent a locker. A good rule of thumb is that if you can't afford to lose it, don't bring it with you.
