Any toys for 16 mo old that involve fastening/unfastening?

My 16 mo old boy loves fastening/unfastening straps, plugging in cords, etc. Any toys that would let him do that kind of motion but are safe? Buckles and button toys are too hard--needs to be something easy. He does have MegaBlocks/Duplos already and he likes them, but I'm looking for something else.