A calculator keeps my DS busy for a while, it's just as interesting as the phone or the remote, but not off limits! I keep 2 of them in the living room, but I keep them on the coffee table so that DS does not think that they're toys...this way he feels like he's getting away with playing with an off limits object...even though it was planted there for his amusement! Ha!
He also loves his Latches board from Melissa and Doug. It's got all different latches on it and you can open and close all the little doors. It's really for a 4 year old, and the latches are made of metal, but he LOVES it. Very montessouri...
Happy entertaining!