I would say just give the damage, xp, and armor elixirs for days 1-4, and then get a guaranteed combo elixir on day 5. It doesn't have to be a 3x, just some guaranteed combo (most likely 1.25x or 1.5x). That would make sure people log in to get to day 5. The vanities are cool, but it has been the same ones for a long time.