As a kid I had Mr. Wizard’s Experiments in Electronics and it was one of my favorite things to play with. The set came to me second-hand, from some neighbor who gave it to my Mom to give to me (they knew I had an interest in science).
I remember opening it and feeling the science coming off this thing. It came with a book that explained what to do with a set of modular blocks. These blocks were incredibly ingenious. They were transparent, so you could see the electronic components inside of it. They also had magnetic contact plates so they could be connected to other blocks. When you connected the correct blocks to each other, you would complete a large electronic piece. It was a brilliant toy!
I am not sure what happened to my set, but I tried to hunt it down a few years ago and failed. Lucky for me, there are plenty of the sets for sale at reasonable prices.