For the first 13 years of my life, I lived in rural Pennsylvania. We literally lived on the side of a mountain. I think there was one other kid about my brother's age who lived close enough to play with, but he was a little strange so we didn't play with him that much.
As a result, my brother and I had to entertain each other. Not such a hard thing, really. After all, we had acres of forest to roam around in, a dilapidated farm house further up the mountain to explore, a nearby creek (that my brother almost drowned in once), a monstrous tree house my dad built us and about 30 cats and one toy poodle (who was perpetually annoyed that the cats out-sized him) to play with.
My mom was big on trips to the library, but those books only lasted so long. We played a little Atari, but for the most part our indoor fun revolved around Legos. It became a kind of challenge for us and my dad to build whatever was on the outside of the box before we just did our own thing. Without fail, we perfectly replicated everything Lego put on the boxes.
I have to say, looking back, it's no surprise what a good job my dad and brother did building those massive constructions considering that they're both custom home builders now.
What toys bring back childhood memories for you?