Pick a start and finish line across the room and cross it as different animals.
Bear walk on hands and feet with a straight back. Crab walk on hands and feet facing upwards. Frog jump from a squat. Do each one with them.
Two or three crossings per animal is plenty — these are genuinely hard and fatigue arrives fast.
Bearing weight through the arms builds shoulder-girdle stability, which is the foundation the hand needs to write from later. Crab and bear walks load it far more than any tabletop activity, and children accept the effort because it is a game.