Vinegar and two different amounts of baking soda in plastic soda bottles with balloons.

Video of the demo in action https://youtu.be/GQbULuidNKg

Two 500ml PETN soda bottles of the same make, split a bottle of vinegar between them.

11" balloons are pre-inflated with dry air, with care taken not to stretch the neck of the balloon. Into the balloons with a funnel go one, two teaspoons of baking soday. With 250 ml of vinegar, that's like six liters of gas potential if one carbon dioxide comes from one acid hydrogen ion.

Tap the baking soda powder down away from the neck of the balloon. Stretch the neck and place it over the top of the soda bottle, taking extreme care not to get the powder into the liquid. (A tiny bit is recoverable, more is not. Start over.) Once the balloon is in place, with the roll at the end of the balloon neck over the flange of the bottle, and no parts overstretched, and the baking soda in the body of the balloon, slumped at the side of the standing bottle's neck.

Most common failure mode is where the stretched parts of the balloon where it attaches to the soda bottle are easier to stretch more than the rest of the balloon. That usually ends up venting the gas. So pre-fill the balloons so they are easier to inflate the second time, and use dry air and not lung power because the baking soda has to be dry and flow, not stick in the balloon. Other option is to tape over the bottom of the balloon. Again, take care to keep the baking soda in place.