I'm buying Maxell UR 90 Normal Bias Audio Cassettes, which claim to store 90 minutes of high-quality audio per side.
I'll be encoding my data with minimodem, as suggested by iffybooks zine on this subject. Minimodem converts files to modem noise and vice versa using FSK (frequency-shift keying) at a variable rate. I'm expecting to get reliable data recovery at a rate of 2400 baud. At 1 bit per frame, that's 2400 bits per second. That means that with 90 minutes per tape side, I'm looking at somewhere around 1.04 MB, a little less than a floppy disk. reddit claims that with good audio equipment I could get this up to about 135 MB.
So, what to do with my one megabyte of data? A good candidate is ZZT, which is still maintained by its community despite being published in 1991 and selling its last official copy in 2013. ZZT is now freeware and its source code has been thoroughly reverse engineered by computer witch @asie, who is awesome and if you're reading this you should check them out. I used to write games for this system myself! The engine is 48 kB, and the maximum size for an unmodded game is 20 kB, well within my parameters. I could write my own game, but I'll probably use Galactic Foodtruck Simulator 2099 as my test subject, a ZZT modgame released by @WiL in 2023. The game takes up 471 kB when ZIP compressed, and comes with a 73-minute soundtrack, so I might be able to fit the entire game on one side of the cassette and the soundtrack on the other.