Lewis Carroll Epstein in his book Relativity Visualized has developed several marvelous illustrations curved spacetime. Art has a copy of the book and model transparencies that you can curve and flatten out on the overhead projector to show:
A description of 4-Dimensional space
See Local Inertial Frame (Gravitational Acceleration) [1]. This demonstration is equivalent to the Monkey and Hunter [2] example. A large aluminum frame is cranked up and suspended by a magnet. Two guns on one side of the frame are aimed in straight lines through holes in an intermediate Plexiglas sheet at target pockets on the other side of the frame. If the guns are fired while the frame is suspended, the projectiles travel in parabolas and bounce back from the plastic sheet. But now the guns are reloaded and frame is released to fall. Another switch fires the guns as the cage goes into free fall. When the falling frame is stopped by a "linear decelerator" (a shock absorber) at the bottom, it will be found that the projectiles reached the pockets. In the falling frame, they traveled in straight lines, obeying Newton's First Law.
Our device was designed by Dr. R.E. Berg of the University of Maryland (A.J.P. 48, 310 (1980)), and built by the UCLA physics machine shop.
Attention can be called to the well-known Monkey and Hunter [3] demonstration as an illustration of the Principle of Equivalence (POE). The impressive point is that no complicated ballistic calculation is needed. Consider a reference frame falling with the bullet and the monkey. (Imagine a frame released the instant the bullet leaves the muzzle and the monkey starts falling.) In this frame the bullet is initially directed at the monkey and travels uniformly in a straight line toward it. By the POE this falling frame is truly inertial and the fact that the bullet hits the monkey is an immediate consequence of Newton's First Law. (A. Huffman, A.J.P. 48, 314 (1980))
Two very simple demonstrations of weightlessness when falling are described in Weightlessness (Gravitational Acceleration) [4].
This is a 16 minute NASA video of physics experiments in the orbiting laboratory of Sky lab. Weightlessness is well shown, and the rest is a physics 10 level discussion of gravity, satellite motion, and illustrations of Newton's laws in weightlessness.
Links:
[1] https://demoweb.physics.ucla.edu/node/411
[2] https://demoweb.physics.ucla.edu/node/456
[3] https://demoweb.physics.ucla.edu/node/387
[4] https://demoweb.physics.ucla.edu/node/414