First of all, if you have attended a Global Day of Coderetreat before, you can make a better use of your time, so please stop reading here.
Every year, a global event takes place on a Saturday in dozens of locations around the world. During this day, lots of enthusiastic developers gather together to improve their craft and learn tons of things.
The format is actually quite simple: a challenge is proposed to the attendees at the beginning of the day, and they have to start implementing a solution. There are only three constraints: you must work in pairs, you have to use TDD, and you don't have much time to finish your implementation.
Maybe you're thinking how this can be possible...we have a whole day, haven't we? Well, that's true, but the day is organised in short sessions (45 minutes each). In each session, pairs rotate and they start implementing the solution again...from scratch. Also, you have to throw away all the code you just wrote in the previous session, yikes!
You can try different approaches every time you start over, and of course you can change languages, IDEs, etc. In fact you should try to use languages you don't know, specifically you should find people who know that language and pair with them. Pair Programming is one of the best ways to absorb knowledge very quickly, and you'll hardly find a place having such a diverse mix of skills and the will to share them.
I lied a bit before when I said there were only three constraints. The facilitators usually add different additional constraints in every session, because...well, because this way it's more fun :).
You may find that in some sessions you don't learn much but you teach new things to your pair. That will give you an amazing feeling too, and hey, sometimes teaching things is the best way to refresh what you know, challenge your assumptions...it's clearly a win-win.
If I haven't convinced you of attending yet, I'll give you one final reason: the atmosphere is fantastic, and I'll tell you why. The event happens on a Saturday, and it starts very early. Everybody in the room has woken up very early to be there on time, and rest assured that their intention will be getting the best experience and enjoy the day as much as possible.
I'm writing this post in 2017, and this year the event will be on November 18. So you have plenty of time to find the one closest to you. Of course I would be happy to read your thoughts in the comments section after your experience!