Micronauts were small (sometimes large) plastic figures that had interchangeable parts. They had holes in their body that various pieces could plug into. You could literally take the wings off of a vehicle and plug them into one of the people. I think the whole concept was ingenious and lead to endless possibilities of combinations.
I did not have any of these but my cousin did. He probably had every single Micronaut vehicle and figure that was produced. His parents even built him and entire Micronaut city one year for Christmas. The city was so big it was probably the size of two ping pong tables. It even used different household items like a two liter plastic bottle transformed into a lite rail system.