Reading with interest, I mistakenly shared this with my party's sorcerer, who's now cackling with glee.
Question… Is using a magic item that casts Fireball a 'casting a spell' action, or a 'use item action'?

— CyberNook 2020 (@NookHarper) December 20, 2019

Activating a magic item is its own action as described in the DMG (page 141).

Sorry Thieves, y'all can't Fast Hands it.

— Dan Dillon 👥 (@Dan_Dillon_1) December 20, 2019

Reading the DMG 141, it seems as though some magic items allow you to cast a spell, so the action would be a ‘cast a spell’ action for some magic items, a ‘use an object’ action for others Negative. Activating a wand of fireballs, for instance, is still using your action to activate a magic item, even though you are ALSO casting a spell. You aren't using the Cast a Spell action.

— Dan Dillon 👥 (@Dan_Dillon_1) December 20, 2019

Thanks both.
My group’s Sorcerer is currently grinning like a maniac, and I’m having to plan for napalm death Good luck, and don't forget about the Dodge action!

— Dan Dillon 👥 (@Dan_Dillon_1) December 20, 2019