The treasure chests can be bought from the general/supply store in the holiday world when they go there for World Adventures, an expansion pack in Sims 3, which you might have seen in the Plunkett's stories if you remember those. They can be recoloured all over. They only take up one, or maybe even half a square, and seem to be bottomless. They don't need to actually open it to put stuff in and out, you can just click on the options or ... is there a right clicking, something like that, as you would for the fridge. And if you put it in the family inventory with things in it, they stay in it, not like a bookshelf.
Absolutely anything can be stored in those treasure chests. If you can grab it with a hand and drag it in there, it will go in there. Cars, workstations, insects, rocks, food ...
I wonder if the kids' toyboxes do the same thing. Or the Scrap Metal working bench. I seem to be able to put odd things in the quick grow station fertiliser chamber but not use them, and there's a limit to 3 things.
Of course there's their inventory, and the family inventory for the buy-mode things.
