I was going to suggest you can observe Jupiter's satellites and calculate their orbitals, but then I realised you have a dob, and they aren't too crash hot for fine measurements.
Though, if you can take pictures, then you can do it that way, and knowing the size of Jupiter's planetary disc, you can work out how much the moons are moving each night, which would give you some idea of how fast it is moving - just like the forefather's of astronomy!
Even if you fail to work it out accurately, it is /still/ a good project, in that you can document what you did wrong, etc. Science is more than about getting the right results, its also about learning what no to do.
As they say, you are an expert when you have made all possible mistakes at least once
Also, remember you have far superior optics and time keeping than the first people who calculated the periods of Jupiter's moons, not to mention a wealth of information and a great deal more computing power.