Fill the test tube with dry ice. Place the balloon over the end of the tube. As the dry ice warms it will change to a gas and expand the balloon. The balloon can be made to blow off the tube if heat is applied to the test tube rapidly enough. An alternative is to inject the CO₂ into the balloon with the modified syringe.  Insert the syringe far into the neck of the balloon to make this work effectively. 

Liquid Nitrogen can be substituted for the CO₂ as it expands about 1000 times when going from liquid to gas.  Realize that this is different from the CO₂ as the CO₂ is going from a solid to a gas, whereas the liquid nitrogen is only going from liquid to a gas.  You may speed up the expansion process by placing the balloons into a pan of warm water. 

You can make a small explosion by putting CO₂ into a film canister and then setting it top down on the table.  When the pressure builds sufficiently, the canister will "pop", shooting the body of the canister into the air.