That's easy - write a book or write some music - same thing.

- it isn't based on any short term trend... yep
- it doesn't need to target multiple markets... yep
- it is something that will be purchased many times by the same customer ( repeat business )... yep
- it needs little work/labour to be operated... nope
- it can be advertised easily by words of mouth ( wow factor )... yep
- it needs almost zero investment... yep
- it can be operated from almost any location, any country... yep
- it doesn't rely on anyone else than you... yep
- it's not season-dependant... yep
- it's legal ( obvious )... yep

It costs next to nothing to do/setup, appeals to lots of people (if you craft it right) can be done in any country, doesn't rely on anyone else (except your publisher).