LittleBits are awesome circuit kits that make playing around with electronics simple and fun. If you'd rather make your own to see just how they work, Instructables user JackANDJude shows you how.
The basic premise of littleBits is that you get a box of modules that can be easily connected to each pother and to real world items to form more complicated electronics. JackANDJude's DIY project is basically the same thing, but since you're building the components yourself, you end up learning a bit more about how they work.
You'll need a soldering iron, hot glue, a lot of wire, and various other small parts, but you should be able to put everything together fairly cheaply. Head over to Instructables for the full guide and a rundown of everything you'll get.
DIY littleBits [Instructables]