[KenWittlief]

Personally I think the most usefull one you can get is the gyro sensor, it will allowyou to measure if your bot is turning, or how much its turning, or how far its actully turned

I cant imagine doing auton mode without it - most bots, if you turn the motors on, they drift to one side or another, and when you turn its hard to dead recon to a new heading

the gyro resolves all those problems with excellent accuracy

only thing is, its not a simple device to use - but if you have 3 weeks you can get one and play with it on your EDU bot - the best source I found is Avent (electronics distributor) - they had them in stock last time I checked for $50

search on 'gyro' or 'yaw rate' here in CD and you will find several threads on the subject, and how to use them - I think the part number (top of my head) is ADRSX150EB- something like that - its in the other threads.