Really simple project:
1) Put vinegar in a beaker,
2) Put baking soda in the beaker,
3) Cover the top of the beaker so as to trap all the CO2 gas in the beaker,
4) Once the reaction has finished, "pour" the CO2 gas out of the beaker and on top of a lit candle flame.
5) Explain how the CO2 was produced in the reaction, why you're able to "pour" the CO2 out, and why the CO2 extinguished the flame.