I'll echo the two most common ones already noted: music and language. Where music is concerned, I've owned mandolins for a few years now and have made little progress; I'll play for a few weeks, and then they'll gather dust for a month or two. This year I started off quite well - I learned a bunch of new tunes. However, I then decided it would be best to switch from directional to alternate picking, which essentially meant starting over and re-learning everything from scratch. I found this so frustrating that I have yet to properly go back to the mandolin. I've also had a friend's bouzouki for the past few months and haven't learned anything on it. Language is slightly worse; at least with the mandolin I've made an effort. At various points I've wanted to learn different languages - Norwegian, Spanish, French, Polish, Italian, Russian. I did French back in school and studied Spanish in my first year of university, and of course learned Irish from way back in primary school. All three I've at various points really wanted to learn properly, but never actually gotten around to doing anything about it so far. There are lots of other things I think about learning - how to program, how to knit, how to ride a motorcycle - but I have yet to consider them as properly as the above. ("Properly" used loosely here.)
I think music and languages have that one really important thing in common: once you've learnt one, picking up more is a lot easier (well, with languages I guess it depends). I've been playing guitar for 10+ years now, and I'm not especially great at it (I know people who started six months ago who are insanely better than I am) but I enjoy doing it - and because I'm able to play that, over the years I've been able to pick up the ukulele, bass, violin, keyboards, et cetera. Mandolin is next on my list.
Music and language also have even more in common. Here's excerpt from an essay I did in the first year of my degree:As language is such an essential thing for humans to learn, infants as young as eight to ten months are starting to determine which sounds are linguistically significant (Kuhl, 2006). Research show that as people grow older and fit into their culture and language, this ability to phonetically differentiate between word and letter sounds becomes more difficult. As music requires careful listening to individual notes and their rhythmic values whilst language requires listening for individual phonemes combined with the intonation, both end up being processed by the brain in similar ways (Kindermusic, 2011). Stanford researchers found that music listening training improves the brains ability to differentiate between fast and continually changing sounds. This is a vital skill to enhance for phonemic awareness, which is important for acquiring a foreign language (Gaab, 2005). Studies also show a high degree of correlation between phonemic awareness and pitch discrimination. “The ability to perceive slight differences in phonemes appeared to depend on the ability to extract information about the frequencies of the speech sounds”. Researchers have suggested that singing is a great way to learn language as it molds these two abilities into an effective method of learning (Lamb, 2007). This not only shows music's ability to calm and improve attention in students, but also the ability to be a whole method of teaching together. So the areas of the brain that are also used to process language are being activated through the listening of music, allowing one another to work in harmony gives was to a much faster learning process.