I recently designed an enclosure for an ultrasonic levitator kit that worked out really well.
The levitator has two ultrasonic transducers that form a standing ultrasonic wave between them. The shape of the waveform (which is really just a sound wave that humans cannot hear) is such that it creates alternating areas of high and low air pressure about 8 mm apart.
Small styrofoam balls or tiny bits of paper can be easily suspended inside one of these pockets. The ball shown in the photo here is about 2 or 3 mm in diameter. This model is not powerful enough to suspend drops of liquid, but small bits of paper or expanded polystyrene (styrofoam) are fine.
The ball is not being suspended because air is blowing upward like a fan; there is no real air movement involved at all. The small ball (or small piece of paper) rests trapped between an area of high and low air pressure, like a little pocket. These stable pockets are roughly 8 mm apart, so the object can be "bumped" up or downwards.
It's neat and educational. The kit I bought is available from Tindie here and the enclosure I designed is hosted at PrusaPrinters.org. The enclosure is open on the bottom, so dropped objects just fall right through and don't get caught inside the device.