I always leave the maid a soft drink that somehow ends up in my possession after a flight. I also make sure that the room is as clean as possible when I leave. I dont know if they ever drink what I leave them but most hotels charge a 1.50 for a coke, so maybe that is something they enjoy, maybe not too. On a side note, while on vacation I leave a 20 on the last day. But according to some I must be a rich FO