I would start by making a shopping list, go through the tips on these boards and pick up everything you need such as ponchos.
As far as packing, here's what I did. BTW, my DH thought this was hilarious, he doesn't understand my organization. I wrote down where we were going each day of our trip on an index card, then I laid them out in a row on the floor. Then I made a row of clothes for each day for each person. After I had the outfits I put them in ziploc bags (labeled by day and initial). When their row was complete I put them in their suitcases. I made sure that each person's bag was completely packed with jammies, sandals, hats, etc. before I zipped it up.
The next day I packed my bag for the parks and the entertainment for the drive down and chargers, batteries, etc.
I'll pack the toiletries last.