You could do the separation of gases like he said. It is an incredibly simple reaction.
Get a sealed reaction container, usually a Jar like shape. Make this roughly about 1 liter in size. In the top, put a stopper with a hole in it. In the hole, put in a piece of rubber tubing. Have the rubber tubing about 3 feet long. Next to your Jar, fill a tub with water. Fill smaller gas collecting bottles (make sure they are gas, they have thicker walls so they don't break) fill them with water and put them upside down in your tub of water. Take the rubber hose, and set it at the bottom of your tub. Obtain a square, flat, piece of glass. Set this next to your tub of water.
Now for the reaction. Fill your jar with about 200 ml or so of HCL, probably about 3-5 Molar would be alright, that way it isn't too strong. Obtain pieces of Zinc metal, very easy to come by. Your reaction will be this:
HCl + Zn --> ZnCl + H2
You can balance it. As you can tell, you produce Hydrogen gas as well as Zinc Chloride, a simple salt. You add the Zinc pieces to the HCl in your jar, and then close it with the stopper quickly. You will soon see bubbles of air coming out of your hose that is submerged in your water bath. Take this hose, and insert it into one of the upside down bottles in your water bath, you may have to hold the jar a few inches above the bottom. You will see the gas collecting in your upside down gas collecting bottle. This gas will slowly displace the water, and fill it with Hydrogen. Once it is full of hydrogen and leaking out of the bottom, take your flat piece of glass, submerge it underwater, and cover the bottom side of your jar. Next, remove it from the bath, still upside down, with the piece of glass covering the bottle. Take the bottle, and slide it off of the glass plate onto the table QUICKLY. This will keep as much Hydrogen as possible in the bottle.
Repeat for a few more bottles. Now you can take a burning splint, which is like a piece of wood you lit on fire, then blew out so that it was glowing. Pick up a bottle from the table, hold it tightly, and insert the glowing splint into the bottom of the bottle. You should hear a pop, and your hand will fly up a few inches. If you look into the bottle, you will notice that Water has formed inside it form the reaction. You can have some fun with this now, see how it changes if you take the bottle and let some Oxygen get in it for like 10 seconds, so on and so forth. Pretty basic experiment we did in high school chemistry a few years ago.
Or..
You could do baking. Baking powder, used in making cakes rise, is made out of a combination of Tartaric Acid and Sodium Bicarbonate. These two compounds do not react in their solid states. However, when you add them to batter, it is a chemical reaction that produces CO2 gas. This causes baked goods to rise. The longer you let your batter set before cooking, the less your baked good will rise because the reaction has run too far to completion.
That's about all I can think of at the moment.
