Ring of Ringing. Causes wearer’s ears to ring. When removed the ring itself begins to ring.

Ring of perception filter, when not visible (or mentioned recently) creatures forget it exists

Cursed variant applies this effect to the wearer

Here’s few of my goofy items over the years.

Lantern of in-sight: causes any object or entity you’re aware of and can keep a direct line of sight on to glow as bright as a non magical lantern. The lantern itself does not produce light. Had a player use it to highlight someone hiding in a crowd when the party failed to see them.

Jewel of Becoming: when activated the player became a gemstone for 1d6 hours. The rogue ended up exploiting this heavily by becoming a jewel and either having another player sell her or just being in the path of someone. Once she turned back she’d rob them blind and sneak out back to the party.

Immovable ladder: it was a rope ladder but the rungs in the middle and either end were immovable rods. This one the artificer cobbled together in game and I allowed for it. They spent a lot of time trying to figure out how to use it for more than a reliable way down from a second floor but never did manage anything wild. They couldn’t even really use it to go up because someone would still and go climb up there to set the top rod.

Maxwell’s Morning Tonic: a bitter, dark, and slightly oily potion that when drank, counts as a short rest or turns a short rest into a long rest. It also gives you a -1d6 to hit and sleight of hand. It’s just a strong coffee.

I reccomend this series

https://www.youtube.com/watch?v=KYxuJ9ujAYo

Maybe you can find inspiration in The Book of Wondrous Inventions.

Hammer of Dwarf Throwing.

Can only be attuned to by dwarves. As a bonus action, the user may expend a charge to be launched from the location of the hammer towards a target, leaving the hammer behind.

In one of my dungeons, there was a trapped carpet which caused people who sat on it to belive they were riding a flying carpet. It was in fact an animated carpet, so the barbarian who believed he was flying around the castle was in fact just scooting forward an inch at a time

Ring of Rings. When a player/creature wears the ring, all creatures within 10ft/2 spaces hear a bell ringing from all directions, including the creature/player that equipped the ring. The sound grows louder each turn until it is the only noise those creatures can hear.

This sounds useless, because campaign/system specific details need to be added, but it can be used offensively if set up right, and the name is a pun, so that makes it good by itself. Also gives you a chance to integrate bells.mp3 into your campaigns’s ambient audio to annoy everyone.

One I made up for a caster who always missed their rolls was a sentient wizards hat ‘clippy’. “I see you were trying to cast Eldrich Blast, do you need help with that” and occasionally giving them advantage (they really were rolling awfully)

I was bad at RPing it though and that campaign ended shortly after anyway

A bag of beach.

It’s a bag of holding that contains a pocket dimension, with a beach, some palm trees, and a cocktail bar run by an Orc who wanted to get away from all the violence in his tribe.
The characters can all crawl into the bag and the last to enter turns the opening inside out, making the bag disappear in the real world.
It only fits a light-hearted campaign cause it takes the tension out of a dungeon crawl and it’s insanely powerful cause it lets the characters rest, heal and replenish their spells.

I once read an article that had ideas for 100 doors. They were clever. One would only open if you knocked on it. Unfortunately that’s the only one I remember, but maybe you could consider others.

Rod of animal control: it only works on dogs and it only makes them follow you and perform basic actions like sit and give paw.

On closer inspection it’s just a chew treat

It will be eaten if the dog gets a hold of it

This isn’t exactly what you were asking for, but thought I would share a funny story from a few years ago.

I had a problem player, fell asleep during games, made other players uncomfortable with unwanted advances, would say racist things in and sometimes out of character, etc. So I decided to have a bit of fun on what I knew was going to be his last session with our group. He had wanted a ring of invisibility or a powerful magical crossbow. I gave him both.

The ring was not the kind you wear on a finger though, and he had to spend a full minute to put it on as it required some… prep, he was so embarrassed he never even tried to use it.

The Crossbow was hot pink, covered in glitter, and became soul bound as soon as he attuned to it which was as soon as he picked it up. It shot 4 Eldritch Blasts per attack action, and gave a free attack action every turn. It also compelled anyone within 100ft of the bow to refer to the wielder as Sparkle Bitch. Small price to pay, so he thought. Problem is, the Crossbow was a cursed item and after the wielder fired 20 shots, all the damage it dealt would be added up, multiplied by 10 and then dealt back to the wielder in an explosive cloud of pink glitter. I don’t recall the total, but was easily 4 times the players max HP. He was pissed, and that’s when I told him he was no longer welcome at my table.

More in line with your request though: A magical whistle, when you blow it, once per day you can summon an army of 1D6 chickens who will then follow you around until your next long rest. You can have the army attack dealing 1D4 damage multiplied by the number of chickens. The chickens are unkillable, and vanish in a cloud of feathers when you take your next long rest.

Crown of Good Buggy.

Crafted by a demented druid as a gift for a princess on her birthday. It was quickly regifted to a adventurer.

Once a day when the command word is spoken while worn on the head 10 bugs come quickly crawling out of the crown. Make a dex check -10 to see how many you catch (max 10) the rest skitter off.

Each bug lives a day and while alive can be eaten for 1hp healing and provides a full day of nutrition. The bugs taste of raspberry cream.

A funny one i found on Reddit:

Glasses of hindsight: provide a passive perception of 20 of events suitably far in the past

Be careful with magic items, I let my guys buy a trident of fish command… In a desert. But it turns out lots of snakes etc have a swim speed…