when DS was 3 and was in the childrens for a week, I bought a heap of cheap ($1-$2) sticker books and pads of stickers, they kept him amused for quite a while...(I know it was already suggested, just echoing the pp!)
An ipod touch is a lot cheaper than an ipad and still entertaining. But I agree that other things like sticker books and colouring in are also good. Basically anything that would suit for a long plane trip (e.g. movies, matchbox cars - you could even bring a small garage or make one out of a cardboard box, DVDs, magic marker books, stamps, travel games, small soft toys or finger puppets)
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