I have this headcanon that since the TARDIS interior is infinite that all of bedrooms of past companions are still in there, untouched.
So like, there’s Rose’s room with pictures of her family & stuff. And maybe Martha’s has medical books. Donna’s still has all the stuff she brought with her (like the hat box). And I just imagine the Doctor late at night going and opening those doors because he misses them and it’s all he’s got left.