Thee dice inside a clear plastic cube
Dice games are the perfect way to interact with kids when you are travelling, but flat surfaces are non-existent in the back seat of a car, and on airplanes, seat-back tray tables are too small to allow you to roll dice without the risk that they will slide onto the floor where they are nearly impossible to retrieve. The best solution I’ve seen so far is a dice-sized transparent plastic cube containing three tiny dice. Players shake the cube but do not throw it, ant the dice bounce around inside, performing their randomizing in a safe, closed environment. This is just as noisy as regular dice (so these dice aren’t ideal if your goal is to play quietly in a spot where you are trying to be as quiet as possible). Even so, they greatly expand your options.
The dice are nearly free: 79 cents each online. I bought mine from a vendor at a comics-and-games convention. Having used them for some time, I strongly recommend purchasing the kind where each of three dice within the plastic cube is a different colour. That way, when you are only rolling a single die, you can declare in advance, ‘Just the white one counts.’ If you look around on the internet, you can find a number of variations on this theme, including eight-sided and twenty-sided dice within other dice. I haven’t tried playing D&D in the back seat of a car, but in principle, these variations would make it possible.10/26/17