A better programmer how?
Do you want to develop practical skills?
If so, work on projects in strange territory. If you usually do frontend stuff, learn backend stuff. And it's not just writing code. This involves learning some sysadmin tasks, like how to daemonize your code, how to handle config files and logfiles.
And if you usually do backend, learn some frontend technologies. Not just Javascript, but learn some libraries like Angular and Bootstrap. Learn to make web UIs that don't suck ass.
If you want to become smarter, learn smart people languages, ones that will expand your mind. Some examles are Lisp, Ocaml or Haskell. Smalltalk is kind of glossed as a smart person language, but I think it has lots of merit. It's pretty much the gold standard for an OOP language.