tl;dr: MDN doc sprint, August 12-13, online or at local meetups organized by you. Write docs, get swag.
You might or might not consider writing docs to be fun. (I do, but apparently I’m atypical.) However, whatever your baseline for the fun-ness level of writing docs, it’s more fun in groups. It’s cool to know that other people are doing similar stuff at the same time you are. It’s even cooler to get together with people doing similar stuff. This is why we hold doc sprints for MDN.
You can participate from wherever you are. We use the #devmo channel on irc.mozilla.org to coordinate. For added fun factor, you can organize a meet-up in your town for your friends or local web development user group. Or if you’re lucky, show up at a meet-up that someone else organized in your town. See the wiki planning page to read further details and share your ideas.
There will be swag. Make a non-trivial contribution during the sprint, and you can get an MDN t-shirt (or other swag if you don’t want a t-shirt).