AP physics teacher Dan Burns gave a fun demonstration about spacetime curvature in 2012 at Los Gatos High School. Here's the 10 minute video.
Here's another good one from Burns: the "Pole and Barn Paradox":
This is my favorite Special Relativity paradox. It will blow your student's minds. A runner traveling at 0.87c with a 20 m long pole runs through a 10 m long barn. To the barn FOR the pole shrinks and the barn doors can be closed for an instant with the ole completely inside. From the runner's frame of reference the barn contracts to 5 m and the pole can NOT fit with both doors closed! How is this resolved? You can use the slides and associated videos to go though this mind-bender with an added twist at the end.