This is something I started doing a little while ago, and it's now become somewhat of a daily ritual. I filled an empty plastic storage bin with random things from around the house and left it in it's spot on the shelf. Cups, straws, magnets, paper, wooden sticks, toys, anything small enough to fit.
When he goes down for a nap, I trade everything with totally different items than the day before. Now he's getting up and immediately off to find and explore it. I love the look of surprise when he realizes it's all new. To him anyways. :)
I love the idea of open-ended playing with no real purpose other than exploring. He gets to do what he wants with all of it. It's amazing the things his little mind comes up with. Like this tower he's building.