The whiteboards were awesome:
You know the rest of the story - they saw that the elastic bumper transferred more momentum, even though it exerted a smaller peak force, reasoned that time was a factor, too, guessed that the area under the F vs. t graph might mean something, saw that it was just about exactly equal to the change in p (finally, some lab results worked super-well the first time :), and built our model for the change in momentum:
The best part was the appreciation of the analogy between work and impulse, which I think was more than a little due to the IFF diagrams - adding the initial momentum from the initial diagram to the area under the Fnet vs. t graph to get the momentum from the final diagram just makes it click for a ton of kids. We applied it to a sliding box, saw that it was easier than forces/kinematics, and talked for a free minute at the end of class about how physics used to be taught. I described it to them (I needed to, since more than one said that she "couldn't imagine how else you could learn physics") as "throwing a bunch of equations at you and hoping that the concepts emerge from that and stick." It worked for me, but not for most. Reformed methods don't work for everybody either, but it's a huge increase in the proportion that really build an accurate mental model, without a doubt.