The Playdate has an open filesystem. Certain types of online games could be modified to use the old "play by email" save file swapping approach. A server could even be used to make the experience more exciting. This would also make it easier to transition to a proper online format once a networking API is released.
For example, imagine a game where everyone builds a stage. Then everyone plays through all of the stages and the person with the highest score wins, or people get prizes based on ranking. In this case, you save you stage and upload it to a server. The server combines your stage with a few other stages and lets you download a competition pack with stages from other people. You play through the competition pack and upload your results. The server now allows you to download a new up-to-date master account file with any victory rewards to upload to the Playdate.
It is a little clunky, and people could cheat by replaying until they get a good score. (To mitigate this kind of cheating, some combination of randomly generated PIN codes and dates could be used. I can elaborate if anyone wants me to.) Having said that, "play by email" is a tried and true way of multiplayer gaming without a net connection. Obvoiusly, "hotseat" using the same device is also an option.
EDIT: Hypothetically, the Playdate could prepare data as a QR code for transmission to the server. As things stand, files would still need to be downloaded and installed manually on the Playdate though. Also, the C-API does not have a function to generate QR codes, but libraries surely exist.