The Raspberry Pi is one way to get to grips with programming and interfacing, but the various members of the Arduino family of micro-controllers are also great fun to play with, and there’s a huge range of projects you can build with them.
When it comes to small and low cost, it’s probably hard to beat the Adafruit Trinket. Costing just £8 and not even as big as a thumb, it still manages to squeeze in 8KB of Flash memory and five IO pins, together with an on-board power regulator. Program it through its mini USB port – and power it that way too. Or by battery if you prefer.
With support for I2C sensors, Trinket is perfect for monitoring projects. You can program it using the cross-platform Arduino IDE, and it comes in versions for both 3V and 5V logic – just pick the right one for your project – or buy a six-pack.