Make a Straw Waves Model.
It's Very easy to make and cheap.
You will need only :-
about 90 Straws
A couple of meters of Cello tape.
One can demonstrate a lot about waves from this model. Wavelength, frequency, amplitude, how speed of propagation increases as you increase tension etc. And as you are in 11th class, I think there is a chapter about waves and resonance also in Physics.
You can Find loads of other projects like this on Arvind Gupta's YouTube Channel. They are all under creative commons license so you can actually legally take anything from there.