I always brought a small bottle of dish soap, but I'm totally getting the wash and toss things for my next trip!!
My interesting items include:
*Old socks and underwear (I save year around for Disney). Then you throw them away every day no dirties and more room for souveneirs
*Also, all year I collect free samples of toiltries (shampoo, conditioner, body wash, lotions, etc,) ALso tossable so everyday you're making more room for stuff
* I bring small samples of laundry detergent also.
* Hand Sanitizer and wipes are a major plus.
* I bring my own tea bags, bottles of water (cheaper then the parks)
and pretty much anything that you can find that suits your fancy in the lovely travel sections of your local Target

things you never knew you needed