> What are some things with easy entry-level learning curves?

Last year I realized I could memorize every country on a world map and it would only take a few hours over the weekend. Connected it with some wikipedia searches about countries I don't really know over the next few weeks and suddenly I feel so much more knowledgeable about the world and have a new hobby. There's a lot of stuff that everyone has heard of but doesn't know anything about and it doesn't take much time to fix that.