With lasers so easy to get nowadays you could do a few really impressive physics demonstrations with those. You could demonstrate interference, diffraction, refraction.
You could measure the speed of light in water by carefully measuring the refraction angles of a laser beam and working out the index of refraction.
Try simultaneously producing a single slit or double slit diffraction pattern through the same slit with a red and a green laser pointer.
To make a good double slit, place a small square of aluminum foil on a hard surface, glass would be ideal. Take two thin razor blades (or take one and break it in half and use each half so the back is not sharp) and pinch them together with some small locking pliers. (Heavy forceps should work well) Then make a clean single cut with the pair on the foil. You will get two parallel cuts with a reasonably uniform separation. You want to carefully frame the foil. Pulling on it could distort the slits.
[EDIT: I don't need to say be very very careful here, try using pliers to handle the blades since it's so easy to slip and cut yourself here.]
Hmmm... what else? Well you could make a good pendulum (use a rod, not a flexible cord or string), and demonstrate how its period is not quite the same when it swings wider... (the general rule is you can treat a pendulum as a simple oscillator with amplitude independent frequency provided its swing is below 10 degrees).
You will want to make it sturdy and very low friction so that it swing for a long time at higher amplitude.. you can then get a more precise frequency by counting many many swings over a long time. Compare the frequencies when it swings below 10 degrees with when it swings greater than 15 or 20 degrees. Do many measurements and average the results to improve precision. Here's a link to the math...
Large Angle Pendulum Formula
There's a demonstration I do in my math classes to show the sum to product, product to sum trigonometry formulas in action. Beat frequencies and audio interference. You can download free apps that will produce sine waves of given frequencies and phased out of the stereo channels of your mobile device. (Search for "Signal Generator" apps.)
To demonstrate beat frequencies just output two frequencies differing only by a small amount, say 2 Hertz, you will then hear the tone warble at the difference in frequencies.
To do an interference experiment you can then output the two sound channels to a pair of speakers with a specific distance between them (you'll want to do this outside away from any walls or objects that might reflect the sound, say on a card table in the middle of a football field.) Then walk around the them a good distance away and notice in which directions the sound is loudest and in which is least loud. Do this with different frequencies and/or different speaker separation and also with different phase relationships between the two sources if you have an app that let's you control that. This is basically a sound version of the double slit interference experiment.
The math requires the wavelength of the sound which is related to the frequency by the speed of sound. You can thereby use the interference to actually calculate the speed of sound. You might even try comparing the results for different times of day when the air temperature and humidity have changed and see if you can detect a difference.
If you want to get really wild you could combine the beat frequency and interference experiments by outputting slightly different frequencies to the two speakers, you should then observe a moving interference pattern that cycles at the difference in frequency.
Well there are a few I could come up with off the top of my head. I hope they help or at least stimulate your imagination enough to find your own experiments. I don't know your age/school level but I hope I gave enough variety to let you pick a degree of advancement suitable to your level.
