[nittanygirl]

I like the yarn idea. Even having different team building activities. I was a freshman mentor this summer and we had a retreat day where all of the mentors went out in the woods and had to do team building things. We did a lot of cool activities, and although most were based at the place we went, one thing we did were little games. One being the game where you put an object in the middle while everyone stands on lines (in teams on either side:

|-------------------------------|
|-------------------------------|
|--------------O---------------|
|-------------------------------|
|-------------------------------|
|-------------------------------|

Kind of like that with one object for each team. Vertical lines being the people, O being the objects, horizontal lines are just to represent space.
Every person has a number, the leader/ref calls out a number and that number has to run to the middle and get the object and people start passing it under and over (over the head & between the legs) until it gets back to the original runner who then runs it back to the middle again. If it gets dropped you start over. Once teams get comfortable with each other, you scatter them again.

Any games where you can keep shuffling teams, gets people to work together and in the process get to know each other.