My 10-year-old daughter and her friends love playing with the Fort Magic kit. It’s a box of PVC pipes and connecters, along with clips to attach sheets or tarps. You can build all sorts of things with them, from dangerous blow guns (we use cotton balls and tape with a big needle) to clubhouses. See Fort Magic’s YouTube channel for other projects. We’ve had Fort Magic for a over a year and Jane has not yet become bored with it.
Here’s a video of Jane and her cousins showing me one of their creations.
UPDATE (10/2/2014): My kids are still playing with this!