- Release Valve - 
Other Stuff (I'll clean this up when I'm not at work):
- A GPS and a HAM/Packet Radio antenna for tracking, retrieval, and communication with the balloon.
- We could use weather balloons since they are actually very inexpensive and capable of reaching "near space" (between 20km and 100km). A weather balloon would hit about 30km - well above commercial aircraft and high enough to produce some amazing photographs.
- The FAA heavily regulates these things - FAA Part 101. Our payload will need to be very very light.
- A radar reflector as described in the above regulations.