I read GeekDad the other day and Kevin Kelly posted some pictures of a popsicle bridge his son, along with a friend, had built at home.
For around $12 (my guess at what the supplies would cost at my local craft store) the boys built a bridge that was able to span ten feet. In my lab I'd challenge kids to build the bridge and then we'd see how much weight it could support before collapsing. The cool think about glue guns is that a couple of sixth graders can build a bridge (shorter than ten feet) in a single class period. This gives them an opportunity to build more than one bridge and improve their designs.
The first group builds a 1 pound bridge that can hold, oh say two pounds, and the race is on between each group that follows to build theirs stronger and lighter. Each bridge was photographed before testing and then with the important data included was put up on my classroom wall. If I were doing this now, I'd post them on a classroom blog.
I had books and other resources on bridge building laying around the station and while the first group may only have glanced at the pictures, later groups would ask to check the reading material out over night. I know of several groups that met on the weekends to build and test their structures. For best results, I would recommend letting the bridges sit and completely dry overnight (or at least a couple hours). Otherwise, the glue is still pretty elastic and instead of a satisfying break and crack when testing the weight load, the bridge slowly bends to the ground like silly putty.
I had great success with simple bridge building in my tech lab. It taught the kids to use some simple tools. It taught them to problem solve. It taught them to work as a team. It was a fun place to start talking about some basic engineering principles.
I might note that over the years I used other materials: toothpicks, balsa wood, paper, and straws. One year we had a bunch of Knex donated to the lab and I let them build and test the strength of their structures. I should warn you that you will break at least one or two Knex pieces each time you test. Still, I think popsicle sticks and glue guns were my favorite.
The beauty of a project like this is that it, as shown by the pictures on GeekDad, can be done at home. I had a parent tell me her son loved building the bridges so much that she wanted to buy him the same kit we used in the lab for his birthday. I explained that I went to the craft store and bought popsicle sticks and glue guns. He came in a week later and told me that he had a great birthday that year!
[Image captured from GeekDad: Popsicle Stick Bridge: http://blog.wired.com/geekdad/2008/03/popsicle-stick.html]