This time I carried out an idea that I've had for many years, launching a water bottle rocket propelled by compressed air, and landing it gently using a parachute. It's really simple to do, and all the "ingredients" are easy to get ahold of. You can also find innumerable recipes by googling for "bottle rocket".
The tricky part is getting a good seal between the valve and the bottle. I recommend using a car tire valve, which can be bought at any gas station that changes tires. Notice that the valve is not attached to the bottle! It's attached to the launch pad, and only pressed against the bottle by the release mechanism. The bottle nossle can be used as is. I won't go into more details here, but you can watch the video below, which will show you most of the details if you pause at the right parts. You can also look at this PDF with pretty good building plans for a launch pad.