5 Interesting, Inexpensive Hobbies

I'm always on the lookout for free or inexpensive things to do.  Here are some interesting, inexpensive, and little-known hobbies I want to find out more about:

1. Bullet journaling.  Making bullet journals is a surprisingly popular hobby.  Basically you take a blank journal and make it into anything you want.  You can write your monthly calendar in it, add to do lists, draw a habit tracker...and then get as crafty as you like (check the link for all kinds of bullet journal ideas).
2. RTLSDR.  Take your computer and add a cheap defined radio tuner to it and you can pull in all kinds of cool things like satellite weather images and various radio broadcasts.
3. Painting rocks.  Find a rock, paint it.  It's that simple.  Lots of people are doing this now for an artistic outlet, to sell, to publicize their location or event, to hide around town...  Then there are people who are super talented at this like Lorraine!
4. Dorodango.  I haven't tried this yet but apparently this hobby is drawing a following of people who make shiny balls out of mud.  Yeah really.
5. Lockpicking.  As a hobby, not as a criminal enterprise.  There is actually a large community of people out there who like to challenge themselves and each other to learn how to pick various types of locks.  This was even a topic at DefCon this year!