Asklemmy

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

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 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.

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.

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....

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

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.

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.

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

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.

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

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

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

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.

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.

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

Might be worth crossposting to !rpg@ttrpg.network

Fart ring is a great but fun one

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

Cursed variant applies this effect to the wearer

Castle of Not

Cursed castle. Everything within it can only be defined by what it is not.

[Players meet the beautiful princess.]

"A hideous prince does not stand before you."

[Players find a chest of gold.]

"You have not found a rotten sack full of live crabs."

Cursed ring of acrobatics.

Gives the player great acrobatic skill, but sticks to their finger when they wear it. And they can't stop getting around acrobatically. Any action attempted fails, unless it is done acrobatically. Player has normal or only slightly improved stamina.

Player: i'll get my rope and grappleing hook and scale the wall.

DM: lifts eyebrow you think so, do you?

Player: sigh I throw my pack into the air and leap after it. At the peak of its arc, I flip over it, grabbing my grappling hook and flinging it over the wall as I do.

DM: ok, sounds like difficulty of 15..

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

Alchemy Jug

Source: Dungeon Master's Guide

Wondrous item, uncommon

This ceramic jug appears to be able to hold a gallon of liquid and weighs 12 pounds whether full or empty. Sloshing sounds can be heard from within the jug when it is shaken, even if the jug is empty.

You can use an action and name one liquid from the table below to cause the jug to produce the chosen liquid. Afterward, you can uncork the jug as an action and pour that liquid out, up to 2 gallons per minute. The maximum amount of liquid the jug can produce depends on the liquid you named.

Once the jug starts producing a liquid, it can't produce a different one, or more of one that has reached its maximum, until the next dawn. Liquid Max Amount Acid 8 ounces Basic poison 1/2 ounce Beer 4 gallons Honey 1 gallon Mayonnaise 2 gallons Oil 1 quart Vinegar 2 gallons Water, fresh 8 gallons Water, salt 12 gallons Wine 1 gallon