If you really want to find yourself digging into a lot of different features in the Elixir ecosystem, you can’t go wrong with collaborative tools. If you really want to play with the latest hotness in Elixir, and dig into a challenge or two, I’d suggest building a light multi-user blogging system that leverages Phoenix LiveView for real-time Markdown->HTML previews, while allowing simultaneous editing/previewing by two users. Put a DB behind it so you can also play with Ecto. Outside of the 3 lines of JS you need to wire up LiveView, limit yourself to writing 0 additional lines of JS. Do everything in Elixir and eex/leex templates only. Since you’re focused on learning, skip dealing with user auth stuff. There will be some challenge to deal with handling multiple edits at the same time. May as well play with doing DB persistence in your LiveView, without any refresh. :)
Online gaming can be a good option—if building games is your thing. Personally, I find game-based demos/exercises with certain tools to be a bit too far removed from my daily work to help certain concepts really take root.