One of the more interesting experiments you can do above surface of earth is fly balloons in to stratosphere. Fun thing is that whole experiment isn’t very expensive and can give tons of fun. Probably most expensive thing here is balloon itself. The other parts can be used as you desire. Spaceshark have shared his experiment and build details on his instructable. He used Arduino Uno for logging various basic sensors detecting latitude, longitude, altitude, wind speed. He wanted to keep things simple and self contained, because at heights above 60000 ft some sensors start failing, e.g. GPS. Batteries are also another issue due to cold air above.
He used authentic Arduino to make sure it works bellow -40 degrees Celsius. Fake ones can fail even at 0 degrees. Same applies to other modules. Better use original and tested parts. Apart Arduino, he used SD data logger to record all sensor data in portable media. The only thing is missing here – balloon tracking. It can get to heights of 100000 feet and probably will get out of sight. Wind can carry it many miles away, so finding may be tricky thing. Spaceshark points out that this instructable is for data collecting part only, so hopefully he will post another post where he hopefully covers tracking and recording videos.