I always take quart size ziploc bag to the parks to put my camera in when riding wet rides, like Kali or Splash. That way, even if my fanny pack/back pack gets wet, I know my camera, batteries, and memory cards won't get wet. Use a gallon size one for your guidebook/passporter, etc.
On the last day when my kids want to take one last swim, the bag comes in handy, because I can just throw their wet swimsuit in there and pack it up. Or for when you go to AK and want to ride Kali, but want to take a nice dry change of clothes. Once you ride Kali, then whip out the dry clothes and put the wet clothes in the bag.
I put shampoo, toothpaste & sunscreen in one bag, soap in a sandwich size bag, and laundry detergent & dryer sheets in another bag. That way, if something explodes or gets crush, it'll be contained in the bag.
2.5 gallon bags are great for clothes. Hefty makes them with the slide lock so I put the clothes in, slide the bag almost closed, then roll and squeeze all the air out, then totally close it. Works wonderfully and very little wrinkles. If it does get wrinkled, then just hang it on a hanger and let it steam while you're in the shower.
I love ziploc bags. I have a box in the car at all times because I also use them as barf bags for the kids. It's so much easier for the kids and since it's contained, there's no smell or leaks.