Bowling and take some prizes for first and last and build some food into the night. Climbing wall and see if they'll get them helping each other at the site. It'll cost but could be worth it and can be done reasonably quickly and not too much fuss.
OK not the traditional messing around with planks team building, but could offer the 'pick me up' they need and it's a night off for staff to just sit around and socialise or staff can do it as well.
I shy away from the "planks and rope" things as the invariably come with a far fetched meaningless scenario, when all you need is what needs to be done, invent some 'mission impossible' restrictions and none of the other claptrap.