What does he do while he's in the pen? What sorts of things do you do with him, during the time on the floor you mentioned?

My DS loved/loves:
Bubbles
Reading books
Block towers that he can knock down
Scribbling on paper with crayons
Ride on Car
Throwing balls
Balloons - they are still a hit, and were the toy of choice for entertainment in hotel rooms on our recent overseas holiday