make it sonic! - playdate no1 starts game and speakers play weird chirping souns. playdate no2 hears them sounds and parses em to receive the data from pd no1. (it sends back data the same way; through speakers) thus, both can communicate live and wirelessly. viola! you have fake wifi TX/RX.
I'd love to see support for a turn-based API in the SDK. Would apply to a lot of games and would be great to see the little LED flash when your friend has made a new move in a game.
As noted previously, you can make QR codes that go to a URL on a website. You could have it submit data to a web app using the URL/the query string. To prevent cheating/fake data, you can sign a JSON object or string of data with a hash.
That's what I do for BRAAAINS! (https://braaains.game) for the leaderboards. I'm going to do a tutorial/open source release soon to share my approach (though there are probably better ways).