This is "Peter Graves" by William Pene du Bois. The anti-gravity material is called Furloy.
It's a short book though, not a short story. Published in 1950. My copy is from 1972.
The cover image shows the harness you mention (with golf club head covers holding the Furloy balls, and a fishing rod safety tether).
And here's a scan of an interior illustration ...
I'm pretty sure you're thinking of The Faraway Tree, a series by English author Enid Blyton. Books in the series include The Enchanted Wood and The Magic Faraway Tree.
The "pancake person" was probably Moon-Face, named for his round face that looks like the moon.
In the first novel in the series, Jo, Fanny and Bessie move to live near a large wood. One ...