Ask HN: I have 3 weeks off, what should I build?

I'm a software developer, I just quit my job and I start my next role in 3 weeks. I'm looking for a realistic 3 week long project to keep myself busy because I know I'll get bored very soon otherwise...

I mostly do JavaScript by day, but I like to learn new things/languages/frameworks/concepts/etc by night, so send your ideas away! :)