Cheap Ball Pit!
To mix things up a little, I brought in our kiddie pool from the garage and filled it with our ball pit balls (which I bought at a yard sale for $2!) The kids had a blast playing in this off and on all day!
K loved "diving" into the ball pit (he is my, um, adventurous one).
K got so creative with these: they were his "eyes," he pretended they were apples, and one that was broken in half became a pretend juice cup and later a hat. He also at one point decided to throw out all the yellow ones to one side, and the red ones to the other side (sorting!) We definitely had a "ball" with this activity...=)