There are a couple of really good ones. Both are due to the Bernoulli effect.

Kissing eggs: stick two pencils into a corkboard or something, about 2 inches apart, vertical. Get two relatively intact eggshells that have been broken on one end, drained, washed and dried. Paint faces on them. Put them onto the pencils, facing each other, so the lips are about 1/2" apart. Blow gently between them. They should kiss. This is unusual because people expect the eggshells to be blown apart rather than pulled together. If you are doing this for kids, tell 'em that back in the day, ships passing in the night often collided because of this effect.

Floating ball: Rig up either an air compressor or a water faucet to a small nozzle about 1/8", pointing straight up. If it is water, you'll need to also rig up a catch tray and drainage mechanism. Get an air or water jet shooting up. Put a ping-pong ball in the stream. It should simply stay there. You may have to adjust the pressure a bit. The ball should be stable to small perturbations.