A big part of my own area of interest is designing test equipment.
As a young child, I was interested in electronics (ever since I stuck a door key into the live pin of a mains socket 8-O ) but never had the money for test equipment. I was home schooled and my parents had no money to buy stuff like that. The best thing I got was a Tandy (Radio Shack) projects kit - the type with a cardboard box full of components connected to springs, and a bunch of wires.
So, I had to make do (necessity: the mother of invention) with what I could piece together from salvaged gear. Literally from the local rubbish dump quite often. I always wanted to find out what was happening in circuits and most of what I ended up making was stuff to test other stuff.
So, I became a little obsessed with making test gear. I'm also quite obsessed with accuracy and precision.
Sometimes, I'll actually make something practical or just fun. A 300W subwoofer amplifier, garden control centre (pond pumps and heaters, lights, greenhouse watering, etc), a Christmas-tree LED lights sequencer (before they became common), a remote-controlled power distribution system for my server/network rack in the loft, or a multi-channel timer for the kitchen. But still, my biggest passion is test gear.
At the moment I'm trying to figure out how to actually build a dual DDS signal/sweep generator that I've designed. Home made four-layer boards are not really on (I can do three, provided the middle is just a ground plane) so I'm going to break the circuit down into modules right now that I can build onto a back plane.
The electronics magazines are a good place to start for building stuff. I have often used their designs as a spring-board for my own. Everyday Practical Electronics (EPE) is, in my opinion, the best for semi-beginners. It has a good mixture of easy and advanced stuff, and they tend to describe things well. Elektor is more advanced but often uses hard to find or expensive components (I'm biased against them though - I really dislike Elektor's business methods and owners, having once been a loyal subscriber).
So, as I said in my first post - it comes down to what sort of things one finds interesting. It could be electronics for its own sake of course, then anything fits really.