I had been looking at several different varieties of both clocks and voltage monitors until I ran across this. It has clock, voltage monitor (11-16 volts), and a thermometer. I thought it was pretty decent for $27.
Digital 5 Function Voltmeter2005-01-02 Addendum: It's actually a bit larger than I envisioned and I'm not quite sure how I'll mount it yet. I think I'm gonna mull it over a bit. The packaging is a bit odd too. On the outside it refers to big bike toys or something similar but the instructions inside refer only to cars. It also contains both inside (internal) and outside (wired remote) thermometer sensors. While I'm sure it'll work, I'm not sure how weather resistant it will be. I will probably also simply clip the "outside" sensor off.