The Pragmatic Programmer recommends learning a new language each year as a way to prevent our skills from atrophying. I couldn't agree more, and I know exactly which language you should learn next.
Whatever the heck it is that you're writing every day.
Learn that one because I suspect it might just come in handy. Study the little nuances of the workhorse data structures so you never need to read their docs. Be able to split a string, round a number, and forward a method. All from memory.
Learn your server side framework of choice as an extension of this. Display text on a webpage, text that came from the database, text that was put into a database via a form submit. From memory, with minimal code generation. Maybe slide your Haskel book aside for an hour and work on this instead.
Oh, and if you must learn a new language right away I have a suggestion for that too.