Human calculator! You add the sum of four bars rolled like dice 4 four digit numbers, fast than your spectator can type the first few digits on a calculator. Repeat over and over again. You can even you predict the sum on the opposite side!
Place the die in the box and make it disappear. Tilt the box, they hear it slide over. The kids scream because they know the trick but you prove them wrong. The die really has vanished!