My project launched a payload with GPS, camera, sensors and communications to an altitude of 30km. I obtained most parts ready years ago, but only recently had time to finish it.
High altitude ballooning is an emerging hobby, since price of GPS and communications equipment has gotten quite low. It is an excellent hobby for people fascinated by space flight and telerobotics and has many learning aspects — from systems design to electronics design to software engineering. There is also an exciting risk factor, namely, that you could lose your precious electronics if something malfunctions. In this project, many of my interest and knowledge areas came together. Also, I have verified that the Earth is indeed round and that space is black.