I like to make use of sweatpants and sweatshirts because they're warm and comfy and the kids can use them later. When my twins were almost two, I got red sweatpants and sweatshirts and wrote "thing 1" and "thing 2" on white fabric circles with sharpie marker (probably would have been easier to use felt circles). THey were able to wear the sweatpants and sweatshirts plenty of other times until they outgrew them and we passed them down to another set of twins.
I've also got a lot of mileage out of a black hooded sweatshirt. In kindergarten, one kid wanted to be a cat so I stitched black felt ears on to the hood of the sweatshirt and she wore black leggings with it. Then another year her sister wanted to be a bee so I took off the ears and used masking tape loops to attach yellow felt stripes- she wore black leggings and a pair of costume wings and pipe-cleaner antennae on a headband. This year she wanted to be a cat so the stripes came off and the ears went back on
Bunny rabbit could be an easy one too- matching hooded sweatshirt and sweatpants, make felt ears and attach to hood, attach a ball of tuille or a bath pouf to the back of the sweatshirt for the tail