I like to blow some checkout assistants' minds by flashing a tenner when paying for, say, a £5.30 item. Then when they've put £10 into the supercomputer and it says £4.70, rustle up the extra 30p for a fiver change. This technique actually made one person's face implode, proper Derren Brown style.
PS - I know it's childish.
While in th US, my missus and I had dinner with friends and we split the bill of $90.
Everybody round the table whipped out their phones to fire up the calculators. I just said that it came to $22.50 each (four parties).
One of the girls comments:
How did you do that?