I get unreasonably happy when working on a nice box to send someone, especially when I get to savor it for a couple weeks. The open box lives on the bar, and every so often I’ll make something to drop inside, building the mystery bit by bit… 
Currently in the earholes: 
Ooh, looking forward to the package!
This whole experiment is really fascinating, at least to an outsider like me!
I assume you’ll just be sending audio on the tapes? Believe it or not, the lowly cassette tape is quite the versatile medium. Not only can you encode pictures on a cassette tape, using Slow-Scan Television encoding, but you can actually record full-motion video on a cassette tape! I did this myself, here:
https://www.youtube.com/watch?v=FMLyDbyvjQw
Granted, it looks like trash. But it’s a cassette tape; What did you expect?
Fascinating, but I try to tailor media for efficiency (IE: not take 5 years) and suitability for the recipient’s hardware capabilities. I doubt Joe would appreciate TRS-80 cassette saves, though Mitch might. Joe likes lofi video – he plays with low-res analog TV on occasion.
In any case, our common hardware denominator is our Marantz tape decks, vintage field recorder love is what it seems to be about. (: