Students make observations of candles before burning, while burning, and after burning. Their goal is to categorize their observations accurately as "physical" or "chemical". Simultaneously, they are to follow safety procedures (put on aprons and goggles) during the length of the activity. The argument I put to them is this: If I can trust you to follow safety procedures for this small thing, then maybe I can trust you for another lab with an alcohol burner / Bunsen burner / acid / base; if I CAN'T trust you now, then I definitely won't trust you with anything more dangerous and we won't do those labs.
This activity usually takes about half an hour from passing out papers to cleaning up and follows notes on the difference between "physical changes" and "chemical changes."
A large part of science is making careful and detailed observations. Another part is carefully adhering to procedures. This activity allows students to do both in a relatively relaxed environment (I typically turn off the classroom lights for the ~10 minutes of candle burning so they can "see the flame better", but really, it's because candle light is a fun change from the usual sterile fluorescent lighting)