Summary: There are all kinds of things tucked away in the TARDIS, but some have special significance to the Doctor.
The Doctor is a bit of a hoarder, but as his home is a vast space and time ship with countless rooms, what does a bit of clutter matter? It’s not as if he’ll ever run out of places to put the thing he collects on his travels.
He has technology from hundreds of worlds, interesting bits of sculpture, priceless artefacts, and lots of junk. Then there’s the library, crammed with books, many of which haven’t even been written yet.
Tucked away on a shelf somewhere is an antique gas mask, to remind him of a day when everybody lived.