Go flying.
That's what I did today.
OK, so it was my first lesson after not having a valid licence for 13 years, and it was basically to give the instructor (and myself!) an idea of whether I could still remember anything about driving a plane. No horizon, limited visibility of the ground (not enough to know where I was, see the runway from base leg half the time, etc), in fog and cloud part of the time.
The instructor reckoned that as I could fly tolerably well in that crud I should have no trouble getting back up to PPL standard.