I usually find that turbulence on the SYD-SIN leg makes sleep difficult, so the get pi**ed option seems to make sense, as does eating. Alternatively, watch a movie or two, read a book or get yourself Sudoku'd.

Provided you have enough time between flights, it's worth nipping in to the lounge at SIN for a shower, so that you feel a little fresher when it comes to laying out for a kip on the leg back to London.