I haven't used Flask (or Python) aside from one small hack project. It seemed simple and straight forward enough, and very similar to Ruby's Sinatra framework. Personally in Ruby I would most likely pick Sinatra, in Python I'd pick Flash or something similar.

However if you wanna play with languages you're less familiar with, I'd recommend Go, or Node.js.

Personally I find Go really interesting as it's quite different from the languages I normally work in, and it's concurrency model makes it performant without being a mindfuck.

Node.js is interesting as it's Javascript, but extremely I/O performant due to it's evented nature, but that also makes concurrency a bit of a mindfuck at times.

Those are my suggestions at least if you're itching to get your toes wet in some new fun languages :)