The thing that my job is requiring me to do. Trial by fire. But also after the 4th or 5th language and Nth framework, it gets much quicker to pick up new ones.
But I also agree with the last sentence. When you are just starting out in your career, it's nice to have something more complex (I used to write sudoku solvers) to really get the feel of things. I'd never do that anymore though, as most things are similar enough to something I'm already pretty comfortable with.
The benefit is that the design of a blog web app is going to be very similar in a variety of languages that you can compare different frameworks/languages pretty easily. The downside is that if your language is better at non-webapp things, it's not going to be an easy exercise.
On rare occasion I do end up learning something outside of the job, like I learned basic perl writing an IRC bot, but for the most part it does end up being on-the-job.
Also: If I do not immediately have enough paid work, I brainstorm a side project which could feasibly use it.