Try to inject something new into what you're doing. It might be mundane and repetitive but try to figure out something new which you can inject as a learning experience.
I've not secretly added Lisp code to the actual product, but I regularly do supporting work in Lisp: everything from personal tooling that doesn't affect everyone else to research items: prototyping, data analysis and whatnot. Mostly, I use TXR Lisp; a Lisp dialect I made myself that's good for this sort of use.
Here is a recent example: a program which uses the Gerrit web API to extract comments from review items and turn them into output that looks like compiler diagnostics. I can feed that to my editor to navigate through the code and fix the commented issues. It is called ger2err:
This is a great idea. I've often written small chunks of code in my favorite other language (scheme). I find this very satisfying.
Of course, I only do this after finishing the original task.
Sometimes, it's possible to work up prototypes in your favorite other language before starting the main task. This often leads to better code (i.e. less code).