I had a compass/whistle/thermometer attached to my backpack strap, and found it useful upon occassion. I missed the arrows leaving a fairly large town and used the compass to keep going in the right direction and meet up with the Camino again outside of town. Without it, I would have had to do some serious backtracking. It was sometimes also nice to be able to double check that I was going more-or-less in the right direction. The sun works early in the morning, but isn't much help on a cloudy day. Of course, I have absolutely no natural sense of direction, so your situation might be different. If you're one of those people who can sit down in an enclosed room and point to the west, don't bother with a compass.
The thermometer is only helpful if you want an excuse to feel sorry for yourself about exactly how hot or cold it is. :wink: