Wear loose comfy clothes that breathe and choose light colors when possible. I know jean shorts are "in" for girls this year, but forgo them in the park. No heavy fabrics. Think Columbia fishing shirts and light cotton shorts.
Wear a hat if you can stand it. Yes, you can get sunburn through your scalp; yes, you can get skin cancer on the top of your head. (want a picture of the big ole' divot they took out of my brother's head?

)
Speaking of which - sunscreen. We like Bullfrog ourselves. But buy it and use it if you are not used to getting sun on your legs, arms and face. And remember to use it on your feet, your neck, your ears, and your chest if they're exposed.
Drink lots and lots. Non-alcoholic is better, but some people feel more comfortable with beer. Whatever floats your boat (and is legal, of course). You can share your "I drank 3 diet cokes, 2 20 ounce bottles of water and had a mega-'rita today and only peed once!" stories when you get home.
I love neck wraps. You can get these at most atheletic stores and farm stores. They're bandanas with some sort of cooling material in them. DH has a $20 one he bought at Cabelas and I have several $2 ones I bought at the local ACE. But they are godsends when you're out in the heat all day.
Ponytails. Seriously, if your hair is long enough for a pony tail, do it. I have nice, thick hair. I will wash my hair in the morning and put it in a low ponytail while wet. The underpart stays damp and cool until about lunch time when I put my hair in a high ponytail. This year I will have short hair unless I change hairstyles, but I may just let it grow out a little to enjoy a 'tail.
There are water rides in all the parks. . .enjoy 'em!