I had a very active litter not too long ago, and I wanted to provide some indoor-out door toys for them to play on, things that would encourage their developmental skills, while not costing me an arm and a leg! I based some of the items on agility equipment. I would say these items are only suitable for small to medium breed pups 5 to 10 weeks old, they are not large enough or sturdy enough for use much beyond that age. As with all agility items, use at your own risk and ONLY while you are supervising your puppies! I think that playing with these items make puppies solve problems and exercise their bodies in ways that really enhance their development, and any suggestions for more items are welcome. I put them in a fairly small encloser with the items, so they were pushed to explore the tunnel and climb the chair etc.
I bought this child's play tunnel at Toys R Us for $25, I have it tied in the middle so that the pups are not afraid to enter and exit

This is my most creative invention, a cheap $4 plastic chair bought at Walmart. I turn it upside down and put the legs against the pen. This item has a grate on the underside of the seat, and the puppies use the grate for footing to climb up the make shift "A-Frame" and then they slide down the back of the chair! They also jump and climb over the arms of the chair and chase each other around and around.

I bought this 3 level shelf at Kmart for $7 each shelf is about 12" by 13" and it snaps together. I cut the supports so that I could make 3 tables, one is only about 6 inches high, one about 7" and one about 8" high. The pups will hide under the table when they are little, and they will sit on the table when they grow large enough. Be sure not to make yours so hight that it is unstable, they will jump off and be rough on it.