My personal favorite is this "object permanence" activity. Babies need to learn object permanence - the idea that something doesn't go away just because you can't see it anymore. Peek-a-boo is an object permanence game.
There's a slew of object permanence boxes available, online and in stores. My favorites are the Michael Olaf ones, because they're made of wood and are
I decided to make a "free" version of the beautiful Montessori object permanence toy for Gabriel out of stuff around the house, using ideas I'd already put on Pinterest. I took a shoe box I'd previously been using as a treasure basket and cut a hole in the bottom about twice the size of Gabriel's wooden blocks. I rolled out a towel (aka work mat) and showed him how to put blocks in the hole.
He picked on pretty quickly! That was fun. But I don't think he really "got" the concept. He liked dropping blocks through the hole and then picking up the box. Then he'd put the box down and drop more blocks. Who knows - maybe he's already got this object permanence thing down and was improvising! This is a game we'll be playing for a while, I imagine.