Put a hat on the floor a couple of metres away and take turns flicking cards towards it.
Show the grip once — card between two fingers, flick from the wrist rather than the whole arm.
Move the hat further as the hit rate climbs. Cards fly unpredictably, so this stays interesting long after a beanbag would have become boring.
Flicking isolates wrist and finger control from the arm, which is precisely the separation handwriting endurance depends on. The card's unpredictable flight adds a genuine aiming problem that a heavier object would not.