Harry Lorayne's "Memory Magic Square" rocks! It is in "Reputation Makers", page 193. It combines all of the mathematical by-play of creating a perfect magic square "on the fly", but adds an element of a memory stunt on top of that. My personality does not lend itself well to a straight-forward mathematical wizardry presentation (at least, I am not comfortable with that). But the addition of the memory stunt on top of it really gives me a chance to entertain, toss off one-lines, double-entendres, etc.
The routine is basically this: a 4 x 4 grid is shown with each box in the grid clearly labeled with a number (from 1 to 16 in order). One by one, spectators call out a square (by its number) then name an object that gets written into that square. This is where I have great fun going back-and-forth with the audience, getting clarification on the object called out, making silly suggestions, etc.
For the memory phase of the presentation, spectators can call out an object and you say what square its in, or they call out a square number and you say what object is written there. This may not read like much, but try it on your own (if you can!) -- it really impresses folks.
For the mathematical phase of the presentation, a number is selected and written down. Then all the squares are called out in random order by the spectators (using either the square number or its object) and you call out a number to be written down there. When this is finished, you have made a perfect magic square that adds up to the selected number!
As Harry mentions in his write-up, "The more intelligent your spectators the more they will be impressed..." Once learned, you have a totally impromptu routine that can be done anywhere and anytime (assuming you can have pencil and paper handy). I used it regulary for parlor and sit-down close-up venues, using a large "flip chart" and magic marker (a LOT of presentations were then ready for just a few dollars of supplies).
Check it out!