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

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

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

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.

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.

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.

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

Wand of paralyzing. Just tell them that. Then, when they get to use it, they find out that the wand actually just paralyzes itself and just floats in the air doing nothing. DM got me good there just as I had to perform a spell on a guard. Had to aggressively improvise afterwards.

I've seen someone suggest wildmagic wands for a one shot.

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....

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.

Exploding Bag of Eidolon Dust : until the next sunrise, all your players exchange their character sheets clockwise, they have to play their party members character and class until then

We had one that was a jar of peanut butter you couldn't give away or get rid of bc it'd teleport back immediately. It also gave a huge migraine and did damage if you ate it, and it refilled daily

We had a player who was obsessed with caltrops, and we gave him a magical object that let him turn any object that fit in his hand into a caltrop for 24 hrs. He could make like 50 caltrops a day. There were no real shenanigans to be had except he always turned money he was given into caltrops.

In my last game one of my players got a seemingly dubious treasure - an apron. He later found out it was The Apron of Deliciousness (based on Delicious in Dungeon). He could make monsters and other seemingly non-food items into delicious meals. At one point while trapped in a cave he kept everyone alive with spiderweb soup. queue the horn sound

Might be worth crossposting to !rpg@ttrpg.network

Fart ring is a great but fun one

Ring of mirth - allows casting of Tasha's hideous laughter 1/day, but afflicts user with Tasha's hideous laughter whenever a 1 is rolled.

Vaguely influenced by Baldurs Gate 3.

Not an item but I gave my players a talking pig with a Scottish accent. He was also a huge dick. I had stuff planned for it but they killed and ate it.