I've gotten good milage out of messing around with something "different" than what I use professionally. Try something that challenges how you think programming should work.

Some examples I've had some fun with:

Haskell - functional programming will be applicable everywhere, and lazy evaluation will change how you think

Julia - math! Mess around with big sets of data: your country probably publishes big exports of data that you can use for free

Elixir - mess around with OTP and distributed computing

Ruby - delightful syntax, will feel python-y, could try making something in Rails, still the one of the few frameworks where the idea => working prototype cycle can be under 1 day.

Kotlin - statically + strongly typed, try making an android app!