A lot depends on if you're driving or flying...
We fly. I take a bunch of ZipLocs (quart and gallon size) which can be used for food as well as misc other stuff. If you have a lot of electronics, a power strip and/or extension cord are good ideas; so is a nightlight. I also take a pop-up laundry hamper - even if there is a laundry basket, it can be used for other things (corralling stuffed animals, for example!)
We'll buy small things of laundry detergent, dishwasher detergent, dish soap, etc maybe a roll of aluminum foil, and anything else essential, when we're there. Depending on the rental, prior guests may have left the leftover detergents. (It's the polite thing to do, although some cleaning services dispose of everything that's not on the master rental list.) You may need add'l toilet paper, kleenex and paper towels too.
If we were driving, I'd take a tote with all of that, and maybe any special kitchen things you really like that are unusual. (I know someone who always takes a pancake griddle, for example.) The unit will not have toiletries (shampoo, conditioner, soap, etc) other than maybe one set of little travel size bottles, so you might want to take full size of those if you have room.
Have fun!