An inter-house competition sorta thing! Think triwizard tourney/pan-magical games except it would be two houses v. the other two houses! Students from each age group (intro/OWL/NEWT) would be chosen to compete in a number of competitions (something more creative than just dueling or potion-brewing, though :P) and earn points separate from the house points—though the houses with the highest scores in each age bracket should earn points at the end of the year!

I am 100% down for the chosen students being merit-based or evidence-based rather than randomized because it would promote participation in the first few months of school ;)