I found both learning and just writing in Haskell a joy, and I can recommend it full-heartedly. It might be a little hard to get started with ( especially if you're not familiar with functional programming ), but it's really awesome, interesting, and just flat out fresh once you get into it.

That's what did it for me.

For others, it might be retrocomputing or using small systems like Arduino or Raspberry Pi. (Long ago when I thought I might want to learn to fly, I read an article that alluded to airline pilots flying ultralights on weekends; perhaps it's like that.)