Here are some physics demonstrations/activities that can easily be done, assuming that you have the proper materials at your disposal.

1. Reversing Arrows
The video below shows arrows reversing in a glass jar when water is added. For setup instructions as well as the arrows needed to do this download Reversing Arrows.

2. Burning Paper with a Concave Mirror
The video below burning paper with a concave mirror. If newspaper is used it will light on fire and probably have a large flame (be careful). Although, this is risky the kids love it! And DO NOT look into the light being reflected off the mirror. For setup instructions download Burning Paper with a concave mirror.