The box fold or parallel fold is a really simple and effective way of making pop-up cards. I often use this technique as part of interactive book making workshops as it is easy to make but very adaptable. I taught the technique to H a little while ago and she now uses it to make birthday cards for her friends. Last weekend she made the gem below which depicts the birthday boy karate kicking with H and some of her other friends from karate class. I was very reluctant to let her give it away.
There are loads of ways to extend this technique into something more challenging too. http://extremecards.blogspot.co.uk/ is a great resource.