Okay, I may be the only guy who responds to this thread, but I have a suggestion if you take kids (because its something my wife does for me and she says I'm the only kid she has that will never leave home).
The previous posters covered all the major things, but... if you are doing a long day at the park, here is what the wife and I do. We take a bottle of water in for each member of our travel party. In the heat, those don't last long, but there are water fountains around. Some people, including me, either find the water weird, can't drink too much water without flavor, or both.
What we do then, simply, is get some of those single serve kool-aid, gatorade, or crystal light mix ins that are single use for a 16.9 ounce water bottle (does anyone know what I'm talking about?). That way, if the kids don't want water, you aren't paying 4 bucks for a coke every hour when it is hot. A pack of 8 to 10 usually runs 1.99-2.49, and covers us a day at the park. My wife gets the peach tea Crystal Light, I get the sugar free Tropical Punch koolaid stuff.
They come in little tubes, so refill the water bottle and then add in the mix, close the bottle, shake, and bam... kool aid, tea, etc. for one... for about a 1/15 of the price of buying one on park.
It beats packing tons of bottled water and its a way to save money... so you can buy pins... or Stitch-es.
I hope this helps someone... I worry my posts are unhelpful some days (its a Eeyore kind of a day here).
Take care,
J.