Transum have created a fantastic range of games, developed for use on the iPad. I am a huge fan of using puzzles as part of Maths lessons and the puzzles on the Transum site are varied enough to provide challenge for a huge range of abilities.
Many are variations on traditional puzzles, such as the magic square, with the advantage of allowing children to change numbers around easily, as well as being able to check answers for themselves. There are also a number of other problems and puzzles which I haven’t come across before.
Whether you want to use these as planned activities, or to add another dimension to your challenge area, printing QR codes to take children directly to your chosen puzzles is easy using a site such as QRstuff