This is a lab that my physics students (all seniors) do to calculate the angle of launch of a projectile. Teaching and doing science can be a lot of fun. If you measure the time that it takes the balloon to travel and the distance it travelled horizontally you can calculate the horizontal speed (s=d/t). Assuming that air resistance is negligible and that the balloon reached the peak of its trajecory at the half-way point (in time) then the maximum height can be calculated using the free-fall formula (d=1/2*g*t^2). From there the average vertical speed can be calculated (s=d/t, using 1/2 of the total time). At this point trigonometry can be used to calculate the angle of launch (and Pythagorean's theorem can be used to calculate the average resultant speed).