Luigi's Mansion: where to find Mario's items, the Boos and the Hidden Mansion
Quick answers: where to find Mario's five items, how the Boos are found, and how to unlock the Hidden Mansion.
Original 2001 GameCube release, including its Hidden Mansion mode. The 2018 Nintendo 3DS remake and the sequels Dark Moon and Luigi's Mansion 3 are outside this guide.
Illustrated reference

Mario's Hat
One of Mario's five lost items.
Mario's Hat item from the original GameCube game.

Mario's Glove
One of Mario's five lost items.
Mario's Glove item from the original GameCube game.

Mario's Shoe
One of Mario's five lost items.
Mario's Shoe item from the original GameCube game.

Mario's Letter
One of Mario's five lost items.
Mario's Letter item from the original GameCube game.

Mario's Star
One of Mario's five lost items.
Mario's Star item from the original GameCube game.
Mario's items

Mario's Hat, Shoe, Glove, Letter and Star are found across the four areas, several dropped by portrait ghosts. Bring them to Madame Clairvoya's room to hear her readings.
Boos

Fifty Boos escape partway through the adventure and hide in furniture in rooms you have already lit. Use the Boo Radar on the Game Boy Horror: it beeps faster as you approach one. Twenty Boos break the seal to Boolossus, forty open the way to King Boo, and all fifty earn a Gold Diamond.
The Hidden Mansion
Finish the game to unlock the Hidden Mansion, a harder second playthrough. In the European PAL version it is also mirrored.
A Boo that escapes moves to another room: check the Game Boy Horror map to track it.
Picture details
Game artwork belongs to its respective rights holders.
