The bowl.

The 707.

I think sewing machines would count? They certainly got a hell lot more "portable", but the basic design hasn't changed much since the 1880s. Those things are little mechanical marvels

Dildos

nearly

Maybe not perfect upon conception, but after a couple of decades from common adoption, the bycicle really didn't change much. Sure, you can use lighter and more advanced materials, you can add an electric motor to it (though I wouldn't classify it as a bycicle) but you can probably take a 100 years old bike and it would work just as good as a modern one.

The 3.5mm audio jack. It's so fundamentally simplistic from a manufacturing standpoint and circuitry standpoint that any headset you throw at it will work identically without fail (the key innovation being the speakers or headphones where the analog signal is sent to).

Saw a post just today with a 1000 year old folding chair. Looked pretty much identical to the ones used today. Lost the post but kept the picture.

Potato peelers. The ergonomic handle was a big step forward, yes. But the basic design hasn't (and likely won't) change.

The Bic pen. Sure, you can make it better, but then the price has to go up. You can still buy a nearly unchanged Bic pen from any office store for cheaper than any other writing tool, nearly identical to what they looked like when they were first invented.

Microwave oven. It sort of just...appeared, and the design didn't change much.

The pointed stick.

Maybe FM synthesis, it revolutionised the sound of the 1980s and music production as a whole

The MIDI protocal. The technologies that use it have evolved in all sorts of ways, but the protocol has remained unchanged.