Most things that aren't fun. It's so easy to let money slip through your fingers on nothing in particular and end up squeezing fun stuff out of the budget, when cutting back on waste means you can still have fun and squeeze out the waste.
It's remarkable how easy it is to get into habits of buying stuff just because - food items that nobody particularly enjoys but get eaten because they are there, food items that sit in the back of the fridge before getting thrown out, books that look like a good option but never get read, sale items that seem like a good deal and probably would be if only they ever got used, and so on.