My youngest (5) is obsessed with locks and keys. He has a huge key ring with different types of keys, and he will stop and touch locks that grab his attention. He also loves safes, lockboxes, and handcuffs as well.

My oldest (10) is completely obsessed with Star Wars, and has a ton of those little figures, books, etc. He's also obsessed with giant squid, and octopi. He has had these obsessions for at least 5 years.