X wind and drift calculations.
Mental powers only!! No computers, calculators - nothing but brain cells.
Sat at the end of the runway, have been given a take-off clearance which included the latest, up to the second, surface wind report.
How do you calculate if the wind is within X wind limits?
Next question - whilst en-route (or holding at a beacon), you need to calculate the current drift - lets put the aircraft in IMC (so you have no ground features to look at). Remember - no machines (includes radio aids). All you have is wind velocity as given by ATC or the weatherman - and a track required).
Go for it.