Any
math which can be made into a
game or
puzzle (though some of these puzzles can be quite
hard).
Martin Gardner wrote a column on this subject called
Mathematical Games in
Scientific American, including such subjects as
tiling,
conway's
game of life,
fractals,
prime numbers and lots of other
number theory,
eleusis, etc. These columns were reprinted in a series of
books.
Douglas Hofstadter was Gardner's successor.
Most of the older recreational mathematics which was at all interesting has turned into old chestnuts.