Water-filled dumbbells — the load travels empty

Two rigid plastic dumbbells you fill with water or sand: full they have weight, empty they fit in cabin luggage. Weight is set by the fill level, which gives continuous progression instead of fixed steps. Worth knowing: water weighs one kilo per litre, no more. These are toning and rehab dumbbells, not maximal strength — and the cap gets screwed down fully before every set, because a half-turn short is how the kitchen floor gets wet.