Materials: Cardboard tube, large jar, water.
Fill the jar partially with water. Filling it to about three fourths should do, but don't worry about being exact. Lower the tube into the water a couple inches. Whistle across the top of the tube and at the same time raise and lower the tube in the water. Keep trying different heights. At a certain point the sound of your whistling should become louder. This occurs when the height of the air column and thus the wavelength are at the correct value so that the frequency of its vibration matches that of your whistling. This is an example of resonance.
Materials: Two identical tuning forks
Add some mass to one of the tuning forks. You can do this by melting some wax onto the tines of one of the forks, or putting a rubber band over them. What do you expect that the effect on the frequency of the fork will be? Now strike both forks to cause them to begin oscillating. What do you notice? You should hear a periodic rising and fading in the sound. This is an example of beats.