Apples. There's something in bloody apples that keep me well
awake a couple of hours. Did try a few other fruits but they all
didn't have the same effect.
Coffee only works so far - the first few hours ok but after that
it doesn't do any good except to keep you awake to go for the
odd piss down the back. I stick to pure water after about the
first 4 hours.
I wolf down two apples cut into quarters after my 20 minute
enroute power nap. From then I'm good to go till I reach the
layover pub or home.