Cytoskeleton is the easiest thing to look for when you are looking at onion cells.
This is the kind of cell most biology students study first, because it's so easy to see.
Just peel off the sort of thin, tissue like layer under a layer of onion. You can stain it to make it easier to see with any standard plant stain.
They are just in a single layer, so they are easy to pick out under a microscope, and all the cells are basically just the cytoskeleton and the nucleus.
This might not be very exciting for your students, so I would suggest also showing them some of the creatures you can see in pond water.
But, boring or not, this is the best way of showing them the basics of a cell, such as cytoskeleton and nucleus.