Here are 5 items (technically 12), designed by me to make your players go "OOOhhhh, Aaaah, wait, what?"
Ghostpaw's Bag of Trinketization is the peak of portable storage (short of having a dedicated wizard to cast Demiplane).
Two Items for the Mystic class, the Mystic's Crystal and the Rod of Sardior. They even Voltron up.
The Assassin Stone, I think, is the only item requiring Attunement by a Rogue and man will they love it.
Finally, the Prismatic Stones (or the 7 Demon Stones of Po-Alem), seem like fairly rinky dink items until the players start using formations and forethought at the risk of action economy to bust out a couple of the best spells in the game a few levels before they normally could. Don't forget to have your summoned creatures, simulacrums, and hirelings help to fill the spots.
- from the publisher's blurb