Lake Hylia Island
The island in western Lake Hylia, northwest of the Fountain of Happiness (and the corresponding Dark World Warp). In Vanilla, it contains a Piece of Heart. Is normally checked on the way to Hobo, due to the detour being incredibly small, costing only a few seconds, but eliminating the (admittedly small) chance of a go-mode item being hidden there.
Thanks to Stony2103 for suggesting this entry and providing content.