Chemical based hand warmers. They look like tough-built teabags. Get them at any Home Depot or Lowes.
Also McDonalds cheeseburgers are nice. Where I’m at, they’re buy one, get one for $1. I can get 4 of them for $8. When I was homeless, I was calorically restricted and it sucked so much. One day there was a guy handing out McDonalds cheeseburgers from a big bag, and it was the best thing.