When I was a young Scout, in my first Troop, still unable to tie any knot I didn't learn in Webelos, I went on a campout. Turns out on this outing we made a huge tower out of poles and rope. By end of the outing I was hooked and over the years became a geek of all things rope. I still remember the joy of helping build that tower, climbing the tower, helping erect the tower. I can even still see the tower standing there in camp when I close my eyes.
Sorry... Returning from times of yesterday...
There are hundreds of projects you can do with your Scouts with rope and poles. However you do need to know about rope, its strengths and weaknesses, you need to know how to tie knots and properly do lashings. You also need to know what is allowed. For example in the Guide to Safe Scouting it has specific rules on monkey bridges http://www.scouting.org/scoutsource/HealthandSafety/GSS/gss09.aspx about ¾ of the way down the page.
You can find a handful of ideas on things to lash together here http://insanescouter.org/p/226/66/General_Camp_Gadgets.html
Remember safety first and safety always.