Fisher Price makes a quilt designed with teething rings, rattles, crinkly sounding things, a mirror and other things all around the edge. I think there was even a place with a squeeker in it. THe problem with this quilt is it was too small for the baby to stay on, and play.
Look in the baby and toddler toys, especially black, white and red designs. Baby's love faces with simple outlines. Ordinary things are really fun to add. Baby keys are too big and bulky, but you can get a couple that are uncut from the hardware store. Have them smoothe the edges instead of cutting them. Put them on a key ring with a leather fob. Tie on a ribbon stitched to the blanket. It will be a hit.
Another idea might be to make something like the train quilt, but make it more like a town with roads. Make 'trees' using green fabric and cone shapes. The buildings could be the same as the train buildings, just different colors. Add a silly looking car and pick-up truck and you have a great quilt to play with.
Quiet books have lots of pages you could make into blocks. They are designed to be used for quiet play. Bet something there will strike your fancy. Have fun, Gwyn