What major gods and goddesses have visited the underworld? Or is Hades like no visitors, have a purpose or leave.

There was Athena, when she was assisting Perseus in his quest.

Aidon is also friends with Hephaestus, who often goes to him for the best raw materials and they have a nice little barter system worked out.

Hermes, of course.

Zeus has been to the Underworld, but rarely since the War.

Demeter hasn’t been below since the War.

Anything else would be too spoilery so I’ll leave it at that.