Algebraic Notation
When a number is not a number
When a number is a letter
Letters and Numbers

Following on the theme of algebra and letters replacing the unknown number.

Children like to find the missing number or next number in a sequence. This is a natural way of introducing letters as 'unknown values'. Get them putting down their solutions using:

if 1,2,3,x,5, is a sequence then
x = 4

This can be expanded with further notation, by placing a number and letter together, like 5y or 8m. This goes along with factors and multiples.

3y = 6, 4y = 8, 10y = 20, work out y and find 5y.

I don't like the idea of a for apples, p for pears and x for xylophones. Kids latch on to the irrelevance of such statements and they tend to switch off.