Tethered helium balloon is a great idea, unless you're going to combine it with a GPS locator. Anyone found a link for a cheap one of those?
When i was young i always used to mess about doing stuff like this, i once tied a piece of string to a helium balloon and had it hover over my back garden, the neighbour thought it was a ufo, i would have loved to have this cam back then.
Also remember the time i found out you could shoot bangers through the air like mortars using a pipe.