I grabbed a cheap one for my 5 year old with some blank tapes. I remember how much I loved recording my voice, or the TV, and eventually LOTS of radio. Tangible media has more weight than just the physical object. Especially in something as durable as a cassette.

It comes in bursts but when he's into it, he has a ton of fun, The manual nature of it is confusing for him (he's used to instant gratification), like waiting a few seconds at the beginning of the tape so he can record, but something about a cassette makes the whole process easier to explain and, I hope, to understand and visualize.