I forget where I read this tip, but the suggestion was to take spices in a pill container (the one with the days of the week on it) and use a sharpie to label each section. Put in your checked luggage though! This way you have what you need to cook meals without buying a bunch that you will only use a little of.
I think your budget sounds great and I know you will have a magical trip!
I did something similar with spices, but I put them in a bead holder, from the craft dept. at wal-Mart (or Joann's--someplace). The sections were bigger. Also, I recommend you put the spices in a baggie first, so you're less likely to have leakage.
Since the OP likes grocery shopping--and I'm right there with her--let me give more specific recommendations:
I start out with Super Target, because I can also get anything we didn't pack. Exit WH, turn left, then left onto 192. Super Target will be on your left, a couple miles down.
I also love Publix--they have a great bakery. I even got a birthday cake with whipped cream frosting for my DS5's 4th brithday, attractive and tasty. They are on the right as you head west on 192. Both Publix and Target are turns at lights, easy to get to.
I got a Wynn-Dixie shopper's card--it's to the east on 192, and I was less impressed with it. However, if you want to check it out, they'll give you the card on the spot to get the special deals.
Cheap gas is on the right, near the Target (sorry for being so vague). Do NOT stop at the gas station right across form Sherbeth Road!
My gang also likes to eat at the Golden Corral that's a few hundred yards west of the Old Lake Wilson Road light. Added bonus, they have the booklets in the front, where you can get tons of coupons for other restaurants and activities. Cici's is a little further down and across the street.
As to backpacks, etc.: Keep in mind, the more backpacks you (collectively) bring in, the more you have to lose. We had one backpack for all 6 of us, carrying water, snacks, ponchos, and sunscreen. I had a bonus you may not have--DS14 was going on a Scout trip to Colorado the next month, and needed to build his stamina for the altitude. So, to get out of running daily with his sister (DD16 runs cross country and needed to train daily), he carried the backpack. Without complaint. For two weeks. We did take turns with it, a bit, and if we were splitting up to do thrill rides, the backpack stayed with the Chicken Brigade (headed by yours truly). This might work out for you guys if you have a plan, say, Dad carries it for the first 2 hours, then it gets passed to Kid A, then Kid B, etc. Dad goes first because you'll be drinking the water, so it will get lighter throughout the day. I find my kids complain less, so long as they perceive that things are fair.
Sorry for the book. Happy planning!