The aforementioned SIN-EWR certificate. The passenger behind me on Concorde asked if they had any eggcups (he was a collector). They didn't, but wrote eggcup in market on the side of a teacup instead and gave that to him.
I've had playing cards from SQ too and CX like giving me pens when I do an inflight survey (and sent me some cufflinks once which was nicer).