1. Sunscreen: You're going to get awful farmers' tans, shorts' tans, and even sock tans...but at least you won't burn!
2. A writing utensil and an over-the-shoulder bag. These are necessary for drills!
3. SUNGLASSES!
4. Comfortable tennis shoes--really, pick being comfortable over being stylish.
5. A waterbottle. You'll want more than the little cups of water from the general water hole.
6. Random, fun props and items of clothing: you never know when you're going to need them!
7. Your instrument (duh).
8. Pennies for the Penny Wars!!! The section that wins gets to pie their section leaders, so bring all the pennies you've got! Also bring other change, as they count negatively against other sections. :)
9. Yourself. This may seem obvious, but if you didn't get enough sleep last night and your brain is fuzzy, you won't be able to give 100% to the band. Make sure you're not only physically present and ON TIME, but also mentally prepared, whether that means getting enough sleep or fully memorizing your music for that day's music checks.
10. And most important, optimism and energy! Band rehearsals can get tiring, but if you're excited about what you're doing and don't complain about the hot sun, confusing drills, or lengthy practices, chances are you'll spread that positive energy to others around you too! :)