A few from my childhood:
The first one I’m thinking of involves three siblings who meet a fellow who leads them on a trip in their imaginations. As I recall, it’s two boys and a girl, and they meet him by trick-or-treating? Anyways, they all go on this trip in the imaginary world, I think they’re going to meet the ruler, who’s some sort of weird creature. With a sweet tooth - it has a flower growing out of it or something. There’s a train which may or may not have been able to fly, I think there was a boat in there somewhere. I do recall that there’s a sort-of bad guy - I think he’s like a minister or something, and he doesn’t want them to meet the ruler. Oh, and there’s a scene in the real world where the girl imagines her food is really huge, like landscape-sized, and the parents are weirded out.
Second, and more fuzzily, a book with a sort of hidden-valley-with-little-people kind of setup. The main points I recall is that at some point, one of them had gone out into the real world (in a hot air balloon?) and brought some stuff back. Except, they don’t really understand the things: there’s a painting of a tree, and also a family tree, and they switch the names. So everyone plants a family tree in their front yard, and paints little connected boxes with names in them. He also brings back some swords, or knives or something, and the main characters get kicked out of the village for wanting to learn to use them, but they end up fighting off an invasion of mushroom-people. Or something. I think the mushroom-people come through a cave in the mountain.
Lastly, a sort of Mary Poppins-style book, except the babysitter is a man. Adventures include setting up a miniature train with little pills that produce lots of smoke, and the fire department shows up, and a flying bathtub? Something flying, anyhow. Also, the guy says the phrase Bouncing Butterballs, I think.
Come on, dopers, I believe in you! Help me get my nostalgia on!