What a great thread!
Here are some of my favorite things:
A small beach pail- I take one of my DS pails and put shampoo, conditioner, and body wash in it. Then when I get to the room, I unclip one side of the bucket and loop it over the shower rod! It keeps everything up and away from DS reach and he plays with the bucket in the tub. I also use it to rinse him at the end of the bath with fresh water from the spout.
A blister kit- I use the ever handy ziploc bag and fill it with pre-cut rounds of moleskin, first aid cream with pain reliever, and a matchbook that has needles taped under the matches. That way if I need to pop one, I just take out a needle, heat it up and it's sanitized. By pre-cutting the rounds, you don't have to worry about not having scissors in the parks.
A clear jumbo trash liner- I bring one a day into the park and leave it under the stroller. If it remotely looks like rain when we park the stroller, I bag it and it's all dry when we get back to it. Ponchos can blow away and now I don't have to worry. I have also seen people use a plastic drop cloth and clothespins.
Old clothes for DS- He'll be almost 3 for our next trip and I've done this twice before. I pack an old sweatshirt, tee shirt and shorts/pants that have seen better days. I put them in the bottom of the stroller and leave them. If they "walk off" there's no worry and I don't have to tote an extra pair of clothes in the backpack when we park the stroller.
The Pop Up Hamper- No one can sing the praises of this invention enough!
The stop-over bag: When we drive, we usually spend one night each way, so I pack one bag with everyone's stuff for that night in it. Even if I bring in all the suitcases, we don't have to open them up and dig around in them.
Keep those tips coming!