Melissa pointed me to this discussion of favourite rooms in children's literature at Jen Robinson's Book Page.
OK, Maria's tower room (The Little White Horse) and Anne's bedroom (Anne of Green Gables) have been covered, as has the cupola in The Four Story Mistake.
But if we're thinking about the Melendys anyway, how about their first upstairs "Office" back in New York, with the trapeze and all that? Or Clarinda's secret room?
I've always liked The Grape Room in The Moffats. And the kitchen/sitting room in Understood Betsy.
I'd like to be in the Stanton's living room on Christmas Eve, eating mince pies in front of the fire. (The Dark is Rising) Or in Mole's house, ditto, eating whatever they ate (fresh, no canned mind you) with the mice. (The Wind in the Willows)
Someone already mentioned the original Boxcar in The Boxcar Children. I'd like to hang out in the restored train car in Margot Benary-Isbert's The Ark. Or in the converted trolley car in The Trolley Car Family.
I'm not so sure about staying in Aunt Sarah's house in Magic Elizabeth, at least not at the beginning of the book. Maybe after it stopped raining. But Sal did have a nice bedroom.
And once in awhile I'd like to live underground with the Wombles.
P.S. Oh yes--I've always thought I'd like to have tea in Alfie's kitchen.