I say that we continue off of what we have already come up with in the "Brainstorming" topic; what with the puzzle that the champions have to complete and all, earning a piece to the puzzle with the completion of each task. Anywho, as for the first task.
I was clicking around HP Lexicon in search of something to work with for this task, and came up with the idea to have the first task within the castle. To do this, we would have to pull everyone other than the three Champions out of the castle, and have the rest of the school wait outside for the champions to return. Of course, the objective of the task would be to find the first piece to the puzzle by answering a riddle, which they would be given a weak to solve. We would have to provide the Champions with obstacles, I figure, and Jack owns the Marauders Map, so someone would need to put some sort of enchantment on the castle, making aid of that sort useless to them.
OR, we could fallow in the footsteps of J.K. Rowling and put one of each Champions' friends in danger- perhaps one could each be in a different location, locked away with beasts of sorts. For example, we could take Andrew (assuming that it would be too difficult for Chase to play both Chase and Selene) and lock him in an empty classroom with a Manticore or something =P Though I can already see many flaws in that plan.
However, we would have to find a way to incorporate the puzzle into the task, so the Champions would have to make the decision along the way to either find the piece within the castle first, or their friend.
Once again, I'm simply throwing ideas at you in order to get the ball rolling.