Things I've got working so far (Python programs):

1. LEDs (probably always the first try).
2. Buttons, button to turn LED on.
3. Read the CPU temperature (and Halt the processor if it gets over 80C).
4. 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.

I'm waiting for motion and humidity sensors. Also getting a larger selection of jumper wires. To keep sane you need a variety of colors and lengths. In addition, I ordered several multi-wire spring-clip connector blocks -- would have saved me some soldering on the MD80.

What next to buy?

• 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.