Power strips
USB charging converters
Phone (and other electronic) charging cords
Backpack
First AId
Britta pitcher and filters
Coffee Filters
Wet cooling neck towels
Detergent
Flashlights and batteries
Paper towels
Bathing Suits
Pool shoes
Shaving razor and cartridges
Saran Wrap
Aluminum foil
Gallon ziplocs
Quart ziplocs
Paper towels
Scissors
Sewing kit
Swiss Army Knife
Tape (scotch and gaffer)
Tea
Assorted hygiene products
To me the key is realizing that the locker not only allows you to avoid carrying a whole lot of items back and forth, but also allows you to have access to a lot of items we would probably live without but for the locker (eg the Britta stuff).
Pro Tip: For your second trip with the locker, make sure to bring something with you that can cut open the plastic zip ties used to secure the lid. This isn't an issue on your first trip because the box is delivered to you empty and unsecured. Many
DVC rooms have knives or wine openers with cutters that will work, but that's not always the case. The Swiss Army knife or any other tools inside inside your box doesn't help. And you generally cannot bring knives on carry on luggage, so it can be a bit of a puzzler, as many people try to avoid checking bags and even if you do have them and ride the WDW bus sometimes your checked luggage doesn't arrive until hours later and you want your stuff
right now!. Yes, I did not anticipate this issue for our second trip with a box, but I managed to get it open with a wine opener. I probably would have had to ask bell services to come rescue me if that didn't work. Now I make sure to bring a pair of large sturdy nail clippers which are allowed in carry-ons (as long as they don't have an attached file that could look or act like a knife), and those do the trick.