Remember, there are no real rules. Learning and fun are the name of the game. The reason I'm going so far to specify the exercise below is that it should help build some common ground for comparing and contrasting.
Koans are a good place to start. They exist for most languages. Usually there is support in place so you don't have to learn much, if anything, about building packages or other distractions.
List of some koan projects here: http://www.lauradhamilton.com/learn-a-new-programming-language-today-with-koans (somewhat out of date I'd expect)
http://exercism.io/ can be fun to use.
The goal is to validate that things are installed correctly, to learn how to do basic tasks like setting up a test framework.
Pick a kata (or two, if you're ambitious) and complete it. Some reasonably small choices, roughly in order of increasing difficulty: Prime factors kata, greatest common divisor kata, bowling kata, roman numeral kata
There is no need to work it up to the point where you'd be able to perform it fluidly. However, TDD is strongly encouraged.
Learn what it takes to build a package with an executable. It should be installable with whatever tool is common (e.g. stack/cabal for Haskell, leiningen for Clojure, rubygems for Ruby). There is no need to publish to a public package repo.
Build a console tic-tac-toe game.
- Two players: Computer vs. Computer, Human vs. Computer, Human vs. Human (shared keyboard)
- The computer player can be completely naive.
- Bonus points for a "winning" computer player.
Learn how to build a basic web app in a framework/library that is common in the language's community.
Things to consider: What would it take to make it deploy ready (common practice in the community)? How does it influence your design?
Bonus: deploy it somewhere public Bonus: connect to a persistent database
Build a guest book. These were common in the early internet and now aren't seen much.
- Users can leave their name and a message.
- Entries are timestamped and listed in chronological order.
- Bonus: sort options and pagination.