Hold a competition to see who can memorize and recite the most digits of pi.
One memory tool is to create a sentence using words whose number of letters correspond to the numbers of pi, such as the following sentence which covers the first 15 digits (3.14159265358979) of pi: How(3) I(1) need(4) a(1) break(5), mellowing(9) in(2) nature(6), after(5) the(3) heavy(5) lectures(8) involving(9) quantum(7) mechanics(9). Challenge your class to try and construct a similar sentence, or add on to the above example and take it out to at least 20 digits (the next five pi numbers are 32384). Only 20 digits of pi are necessary to calculate the circumference of earth down to a fraction of an inch. For that matter, some mathematicians theorize that the first 39 digits of pi are all that are needed to calculate the circumference of the entire known universe down to the electron!