Thin towels - not thick fluffy ones that will take an age to dry. Those microfibre ones are good.
Suction hooks to hang them on. - though they are easier stuck to shiney plastic or mirror
Use the block to shower in, no point having to fill your fresh water barrel and then having to empty it.
Crocs/flip flops all round for using the shower block
Line the oven base with foil to save splashs and things.
Buy a splatter guard lid for you frying pan
get a drainer like this that can hold alot in a little space.
You don't need as many clothes as you think
Don't forget to shut your hot water tank before you connect the water barrel 
and get a special hose to fill your aquaroll. for some reason all the taps are all the splashiest without.
I put my coffee, tea etc in these which I got in sainsburys/morrisons
Get a folding crate to chuck all the shoes in
I use a big laundry basket to put the clothes we are taking in to carry to the van, then hang up what can be hung and leave the rest in the basket at the bottom of the wardrobe. As long as your dh dosen't ransack it you are careful about rifling through it works well.
Everyone gets a top locker for their own use and thats also where I keep each persons clean undies and pj's.
We have on board all time, tennis rackets, badminton rackets, table tennis bats and boules - all cheapo from argos but fine for what we need. Quite often a site will have the relevant facility for us to use.
I'm sure you'll have loads of fun with it. It's good quality family time. 