Of course, boxes. Any old container too.
Any kitchen utensil they are allowed to have (so not the knives or grater)- the salad spinner is a favorite. They put toys in it and spin, and it becomes a rocket, a magic washing machine, a time explorer, etc.
The toilet plunger. Toilet brushes are out of bounds (I have my limits haha).
The 2 year old found an old socket set that my husband hadn't put back. It's a real crappy one, so we let him play with it. On occasion they get to play with my tape measure and chalk line (apparently they make. the best "cranes").
Plumbing odds and ends have been a favorite lately. Any old fittings, pipe scraps, etc.
Keys that we don't use anymore.
Suitcases on wheels (they get pushed around a lot).
Brooms and mops (I always try and have a clean dry sweeper on them, so they clean my floors when they play).
Old gutters pieces and empty toilet paper rolls make great tunnels.
The dog's kennel (rarely used by the dog) has been claimed as a castle, space rocket, fire station, etc.
We live out in the country and our place has a lot of "treasures", ie bones, bugs and rocks but also stuff inherited with the place like metal scraps, broken tools, engine parts, leaky buckets, pieces of baling twine, dilapidated horse harnesses, etc. For us parents it's a bit of a hassle to go through and sort (and a bit nerve wracking sometimes as we try and get to the dangerous stuff before they do). For little boys it's hog heaven.