I'm seeking for some safe experiments for undergraduates in the electrostatics context. We have a Van-de-Graaff generator in our fundamental physics lab but this kind of devices are useful for demonstration, Can they be used for quantitative experiments?
I assume you have already considered using a cathode ray tube, right? If not, you can apply a constant/static electric field and deflect the electron beam, which is a nice example of electrostatics. The electron will undergo constant acceleration and the problem can be nicely linked with the equations used in kinematics from the previous semester. You can also have the students estimate the force of gravity on the electrons to show that it is negligibly small compared to the applied electric field, which they can test by setting the applied field to zero to see no deflection of the electron beam.
I suppose one could also work with static electricity by using the typical examples of rubbing glass with silk or amber with wool. Then take the charged object and bring it close to an electric field detector (or just have it attract/repel some object of the opposite/same charge).