Here is a simple lesson idea for the teaching of sequences.

Click the photo on the left to start a slide show that describes the task.

What other questions could you ask? How could you take this further? Perhaps you could try building other structures?

There are some nice ideas here, including alternative methods for building and other ideas such as balancing cards.  

Bryan Berg is the current world record holder... he uses a really nice technique for stacking the cards as you'll see from the video on his main page - see below for a screenshot, and here is an explanation.

He states that larger structures are actually more solid due to the weight of the cards; look at the image below!