Knife, always. A few tools, usually. Liquids > 3 oz. Usually spare machine parts for friends who own a house and spend winters down there. Once, a backpack fire extinguisher. Once, a large wok.

It's easy to say "just buy it there", but I don't want to spend any time going from store to store looking for something that I already own and can just throw in a suitcase for free (I get one checked bag for free).