This is very simple, and I'm unaware if the iPhone or watch can tell you of the other's current battery charge, but it would be useful to have an app which notifies you when your iPhone is fully charged, or vice versa, and their current charge, estimated time remaining until full, etc.. Say you are at home and begin charging your iPhone and are in another room, your watch taps you and tells you when it's fully charged, or vice versa.
Another app idea I have and I'm unaware if it's possible, but using the iPhone and watch to measure distance. For example you sit your iPhone down and move away or hold your watch where you want to establish the distance from it to your iPhone, in order to roughly measure a distance. In the future when the watch gets a barometer you could measure height as well.