I was thinking about card sets for my learners, and contemplating using cards they could write themselves in the class. They could then keep them for use in games etc. This led me onto playing cards. I was wondering about explaining the picture cards - King, Queen, Jack - as tens and Ace as either one or eleven. I then realised that they would be good as an introduction to the substitution of numbers for symbols.
The symbols are well established, and wouldn't take much explanation anyway.
K + 1 = 11 for example. This could be examined through games like Pontoon.
Also, as Ace = either one or eleven, it could also be used to illustrate simple algebra with A being unknown, but still simply either 1 or 11.