To build upon the idea that 'magic items should be more than just stat bonuses':
Lightning Arrows: fire one of these and it transforms into a bolt of lightning. Hit something made of metal, or into water, and it'll damage and stun everyone touching it. Fire into the sky and you might attract stormclouds or storm archons [maybe exchange for some other sky creature relevant to your world].
Mithril armour and Glintaxe are fine, just that they are not Dungeon World-class Magic Items. Masterwork, special materials. These things obviously have a place and I'm not saying don't use them, but when awing the players with magic items, aim higher.
Boots of Elvenkind: A pair of finely made leather traveling boots. When walking, your footsteps make no sound at all. When running on naturally uneven ground (forests, underbrush, swamp) you never stumble or trip.
Grasp of Droskar: A left handed black iron gauntlet that changes size to fit any humanoid person. Placing the gauntlet on your left hand causes excruciating pain as you forearm and hand is transformed into a dense black stone. The fingers of that hand cannot move, but your left arm is now pretty much a mace. It's at least as hard to make a dent in as regular stone, maybe it's indestructible. It's said only powerful magic can make the gauntlet come off.
Crown of the Kobold King: This burnished bronze crown is set with various spikes and knotted wire. When you wear it, you are filled by the spirit of an ancient dragon and lord of kobolds. All who see you recognise your royalty. It grants 2-armor, too.
Heartripper Dagger: If the dagger is used against a completely helpless (tied up, completely unawares, etc) the Heartripper dagger pulls the heart from its victim (automatically killing the victim) and you can use the heart for one of these benefits;
* Consume the heart and regain all lost HP and all debilities that the victim did not have.
* Utter a command word and the heart bursts into flame and crumbles into ash and causes the dagger to do +1d4 fire damage that ignores armor for the next 24 hours.
* You may speak another command word and the heart bubbles and smokes. Breathing in the smoke allows glimpses of far off places or of the future.
Most often, just describe what happens, no mechanics. "Your hand is flippin' made of stone, how are you holding those lockpicks?" is completely legit without the specified -3 penalty. That kind of all-inclusiveness is what allows for loads of weird stuff when playing DnD RAW. Better just trust that if the fiction tells us something doesn't make sense, the fact that the emchanics doesn't give it a -5 is not reasin it works as normal. If you can't do it, you don't do it.