End Bosses
Make sure to give the father, the old man, and the magician frequent power-ups as they will fight the Bosses at the end.
Hidden items
If you send Halley, Emmy, or Phillis to secluded area of a level, there is a chance that they might find rare hidden treasure such as accessories or materials. This chance is higher if the area was completely empty (no enemies, CIMA nests, normal treasure chests or switches).