If you want a challenging strategy game for 2 players, then Blokus Duo is a great choice.  It provides all the challenge and fun of the original Blokus game, but is smaller with only two sets of playing pieces.

Both my kids love Blokus Duo, and even though there's a 3 year age gap between them, my daughter has still managed to beat her older brother a couple of times.  So it's a great family game.

The rules of play are quite simple, but play can unfold in an almost infinite variety of ways.  The idea is to fill as many squares on the board as you can with your own coloured pieces.  The only rule is that your piece much touch another of your pieces at the corners - and only the corners. 

The original Blokus game was awarded Game of the Year and Silver as Toy of the Year in 2004, and this new version is already proving highly popular.