We keep... big coffee filters for the big DVC pots (cheap, but a pain to keep buying, or to remember to bring from home), laundry detergent, old reusable mugs (not to cheat the reusable mug system, but just to bring our own room-made coffee out and about with us), disposable cups with lids if we have too much coffee for the disney mugs, an Igloo cooler backpack, a proper knife, utensils, salt and pepper in ziptop baggies, old magicbands, blister-protection/first aid, clothing organizey things that hang from closet rods to maximize clothing storage, paper bowls and plates b/c most often I've been staying in a studio with my cousin and we run out, plastic utensils, scissors, reusable shopping bags for our Publix/Whole Foods/Trader Joes trips, toothpaste, tweezers, hygiene stuff....all the stuff you end up needing but don't want to bring on every trip.
Some keep liquor, coffee makers, clothing, etc etc etc... Those aren't things I feel I need in there, but it's all about what will work for YOU.