Something related to language. I love learning about and discussing languages. From the script to the grammar, it's all so interesting to me.
I'm currently learning Mandarin Chinese. I've pretty much learned Esperanto, so now it's only a matter of picking up unknown words as I come across them; I'm only passively studying it. I thought about learning either Afrikaans or Frisian as well since they are so similar to English, but both are on the obscure side (Frisian especially), so I'm not sure how smart it would be to devote so much time and effort to the language only to find out I can't do anything with it.
The only thing getting in the way of me practicing is my lazy self. I use Anki every day and write vocab words twice, but I do little in the way of writing or reading, let alone listening (my listening comprehension skills are dismal).