Things I've got working so far (Python programs):
- LEDs (probably always the first try).
- Buttons, button to turn LED on.
- Read the CPU temperature (and Halt the processor if it gets over 80C).
- Read an MD80C20 temperature sensor. This gave me some trouble because the waterproof version connected by a 30" cable came with no clues about which of the three colored wires corresponded with the pins on the bare MD80... I finally found it: red=3.3V, black=Ground, white=Data. Probably obvious to most Rpi users. Also, I had trouble connecting the 3 wires to the breadboard. The ends of the wires were tinned but only stripped back about 1/8". I stripped another 1/4" back but the wire was too flexible to make a decent connection to either the breadboard or to the female end of a jumper. So I cannibalized wire bits off an LED and soldered them to the flimsy MD80 wires. Voila, it worked.
- Rpi camera board: $30
- A stepper motor: $5 and up
- A "power tail switch" to switch 110V/500W: $26
- Controls for turning on garden/farm irrigation pipes.