Really had to scratch our heads on this one, all we could come up with were
Buying the wine and soda packages rather than buying drinks individually,
Taking wash powder on as Annie has already suggested,
Taking drink on with us for cabin time
Ordering room service for breakfast - a roll & meat makes a great snack whilst we are off hiking into the hills.
But the funniest one we found, about mid-cruise we start to crave salted crisps/ships, but a packet on board around a pound or dollar on most ships, yet if you order a sandwich from room service, is more often than not comes with them and costs nothing. Plus you get a great sandwich into the bargain
Also on P&O we've often seen 'early bird' dining prices for the speciality restaurants. You still get the table for the full evening, you just had to be there before 6:30.
There must be lots more, and we are always eager to learn
Alan & Katrina