I'm sure I've already shared ideas for 51st birthday or anniversary ("Squad, Engine and Station 51, from Emergency!"), 55 (speed limit sign), and 78 (antique phonograph record).
And of course, the obvious thing for 52 would be playing cards, and if the recipient plays cribbage, the obvious theme for 29 would be a depiction of a perfect cribbage hand (i.e., a 5 for Starter, with the other three 5s and the jack of the same suit as the Starter in the hand, yielding "fifteen-two, -four, -six, -eight, -ten, -twelve, -fourteen, -sixteen, with double-pairs-royal for twenty-eight, and Nobs for 29").
And of course, I can think of a number for which the obvious theme would not be suitable for discussion on a family board.
But does anybody have any ideas for 79? That would work on a mostly-naked pound cake?