Cooking a nice Pi
Easy to manipulate - if you're a creator, that is!
Just subtly change the large-scale curvature of space-time so that somewhere down in the trillionth digit of pi, your digital signature appears. Sagan suggested a string of repeating digits, or the definition of a circle, embedded in the equation of a circle!
To me, that was the best bit of the book and was entirely lost, of course, in the film.