Old abandoned churches, barns, sheds and shacks, which were everywhere when I was a child. (Even the churches.) I broke into a shut-up church as a child and played in it for weeks with my cousins, sister and friends. Then we rang the bell in the bell tower.
Unfortunately my grandfather lived across the street and was the church caretaker; he had the keys, came in, caught us and told us to never go inside it again. He never told our parents either. I only told my mother about it as an adult.
But for a few weeks that church was Narnia, it was outer space, it was the 1800's, it was Oz. It had a raised pulpit for the pastor, and hymnals and Bibles had been left in place on the backs of the pews. It had a stage in the cellar and a working kitchen, but had been closed up for almost twenty years. The bell was tied down by ropes and we cut through them to ring the darn thing.
I used the same church as backdrop for one of my short stories.
My grandmother also lived in an old house (built in 1888) with a staircase that led down to nowhere; I still think about that staircase sometimes. It scared and fascinated me. My inlaws lived in a house built in 1690. It had cupboards that led to secret, hidden closets, and passageways linking it to a newer (circa early 1800's) house. I think about that house, too.