Microwave popcorn. It's convenient enough that I can make it easily any time, but just inconvenient enough that it doesn't all get horked down on the first day.
I find that stove top popcorn is even less convenient (so less tempting to eat all the time), but much cheaper, and maybe tastier in some ways.
Get a ~500 g (1 lb or so?) bag of whole kernels for $3-ish, some oil that you use for cooking other stuff anyway, and salt. Heat the oil on the stove with a few kernels, then when those pop, briefly remove from heat and add more. Make sure the pot has a lid. Keep shaking it side to side to keep the popcorn from burning.
I find it adds just enough oil to taste good, but not so much that I’m eating something super awful for me. (And it’s much tastier than air popped). And I assume you could still add melted butter if you want an extra treat.
I want to get one of those movie theatre style things where the popcorn can fly out of the pot.
I always found these a pain to clean. I had both types. And it's another single use device. I switched to a pot on the stove and never looked back. Bought a special pot that has vented holes on the side so the popcorn doesn't steam and get chewy.