My kids were very excited to be working with Anhydrous Citric Acid. They were a little confused when the acid was in a tablet form. They were straight up baffled to learn that it was something they use and see on a regular basis!
Commonly known as Alka seltzer, the "Anhydrous Citric Acid Lab" is one of the easiest to do with students and reinforces key science concepts. It fizzles, there are different colors, and they get to start thinking about what it means to collect results and potentially make mistakes.
This requires you to have water, food coloring (any color), alka seltzer, and something to time.