So I was driving down to Miami for the workshop, and to make myself feel even more nerdly, I was listening to Richard Feynman's famous lectures. I'm not gonna lie, it was tough to pay attention to his descriptions and drive (at times in somewhat heavy rain) at the same time, especially when he was pointing to slides that I obviously couldn't see. However, it did help set the mood for the coming 3 weeks. One part that did jump out at me, was his introduction to the Law of Conservation of Energy. He developed an analogy of a mother tracking her child's wooden blocks. The boy started out with 27 blocks (I might be wrong on that number, but you get the idea). Then one day, she notices a few missing. However, she looks under the rug, and there they are. The next day, she again finds some missing, but notices that the window is open, and there they are. The next day, she notices a few more, but then determines that a few are were brought by her guest (don't remember the name/relationship, so I'll call him Uncle Buck).
All's well so far, however, the story starts to get some interesting twists. The next day, she notices a few missing, but can't seem to find them, they aren't under the rug, and they aren't out the window. She eventually thinks to look in the chest in the corner of the room. However, it's locked, but this is a sneak mom. She waits till the next the next day that all the blocks are present, and measures the mass of the chest and the mass of the blocks. The following day, again some blocks are missing. She again measures the mass of the chest and finds that it has gone up. She divides that difference in mass by the mass of the blocks, and behold, it matches the number of blocks missing. Thus, she's figured out where the blocks went.
I'll save you/me the rest of the story, he goes on to develop a second "hiding" place in the filled-filthy bath tub (why this over-analytical mom didn't clean it isn't discussed), based on the volume of the blocks.
The reason I bring this up is that I think the story can be tweaked to be a great lead-in. Can it be changed such that it makes the students want to figure out where the blocks went instead of telling them? Could it be some salt to get the students wanting to know where they went? Thus, when you now bring in energy, specifically an energy loss, they begin to think to look for it in other places? Maybe even leaving it to them to determine how to calculate that lost energy?