They float
They explode

Materials:

1 cup cold water
1 tablespoon lye (sodium hydroxide)
1 square foot heavy-duty aluminum foil
12 inch round balloons
750 ml wine bottle, or similar
5-gallon plastic bucket

Procedure:

Dissolve lye in water, funnel into bottle.  Place it in the bucket, preferably outside.
Add aluminum foil to the bottle.
Stretch a balloon over the neck of the bottle.
The balloon will inflate with hydrogen over the next few minutes.

Details:

I find that a square-foot of heavy-duty foil is just about enough to fill a 12-inch balloon.

Cut the foil into strips 3 or 4 inches wide, roll them around a pencil or similar-sized round thing, and drop them in the bottle.  This seems to speed the reaction while minimizing thermal shock to the bottle.

Use more or less foil for larger or smaller balloons.  Or put lotsa foil and add another balloon when the first one is full.  Beware exothermia if you try this trick.

If the neck of the balloon fits loosely on the bottle, tie a string around it.

Warnings:

Do not give these to kids as toys, and do not fondle any popped remains.  The balloon will contain some lye.

* Lye is hazardous.  It dissolves skin rather quickly.  Rubber gloves and eye protection are recommended when handling it.

Do not use an aluminum measuring spoon for the lye unless you want holes in it.

Beware that this is an exothermic reaction.  I have had bottles break from the heat.  If the balloon starts filling quickly, remove it and add cold water to the bottle to slow the reaction.

I recommend placing the bottle in a 5-gallon plastic bucket and adding cold water until it is above the level of lye-water in the bottle.  This will help cool the bottle, and if breakage does occur will contain and dilute the mess.

* Hydrogen is explosive.  If you doubt that, just light one of these babies.  I recommend attaching your match to a long stick - three feet should be enough.  The blast is not particularly dangerous, as explosions go, but can be quite a surprise.

It is illegal in my area to attach a fuse to these and release them.  It is probably illegal in your area too.  Ask your local authorities.  I suspect the answer will be some variant of "NO!"

Have safe fun!