We travelled by car to London (a 4-5 hour journey) with our DS and GS when GS was 4 1/2, and I looked for games that would amuse him. One game that was a roaring success was 'Little Things' by Klicktock (not 'Little Things Forever' which wasn't available then). You are shown a picture of a horse, or a beagle, a camera and so on, and each picture is made up of hundreds of tiny objects, the 'little things'. The game requires you to find specific items from each picture - it might be 10 paperclips, or it might be a list of different items, a basketball, a cup, an elephant, a candle etc. etc. As you find each one you tap on it and it is surrounded by twinkling stars and crossed off the list. Once you have found all items you get a piece of a jigsaw puzzle, and when you have completed enough games to earn all the pieces of that particular jigsaw, then you get to do the jigsaw!
I downloaded this game 5 years ago and it has been a favourite ever since. And I'm not just talking about the children, as my DH and I both play it regularly - it is absolutely compulsive