Build a simple Chladni plate using a 6" x 6" metal sheet, a 2 watt speaker, powered with a 20 gain amplifier and phone. Sprinkle sand on the sheet, and play various sound frequencies to reveal the sheet's assortment of resonant vibrational modes.
Along with the plate demo itself, an amplifier can be made fairly easily with an LM386 op amp. Download instructions for making the amplifier under associated files or can be found here.
Tuning forks are one of the few instruments that resonate with one distinct frequency. Create a workshop with tuning forks with this variety of quick, fun demos ranging from visualizing sound with lasers, to watching energy transfer through pendulums.