The real treasures to find are the tidbits of story and atmosphere to pick up. Anyone that's been in the Dunwich Building, Mama Dolce's, pretty much every Vault, and Fort Constantine can atest to this. While two of the mentioned also include some very nice pieces of gear, the real joy is seeing what went on inside.
***Spoiler country, read at your own risk***
The Dunwich building has a series of audio tapes documenting a survivor trying to stay alive against a horde of ferals, and finding an artifact that is slowly turning him into a ghoul. Mama Dolce's is home to one of the Chinese Military hideouts and is polluted with ghoulified Chinese soldiers and officers. All of the Vaults have something interesting and bizarre going on, like the psychotropic gas pumped into the vents cause ghostly hallucinations, or the lone Overseer Gary given a series of cloning vats to try and cure his cancer. And Fort Constantine has the best armor in the game, as well as letting you set off a tactical nuke against China, lol.
***Okay, I'm done.***
And there is plenty left for you to do once you've beaten the game once, it's almost impossible to have seen everything in your first play, and there is stuff you can do to liven up the experience and see it from a different angle. If you played like a soldier, play next like a diplomat. If VATS made it too easy, play willingly ignoring VATS entirely. (Which is harder than you'd expect)