Empty egg boxes, shoe-boxes, cereal boxes, cardboard rolls, anything not too big (it’s likely to be part of your home for a while but if you have the space, by all means let your children create the next Titanic!).
It’s easier to decorate cardboard than plastic, but there’s no reason you can’t have clean plastic bottles and yoghurt pots too.
Then some PVA glue, lots of rolls of sticky tape, old magazines, paints, foil, old bottle tops, dried pasta, scraps of materials… anything goes as long as it’s safe.
The key here is to let the children take the initiative.