I just had a great idea! Notch commented that he wanted to make cave maps: why not just make a metal detector? It could work 1 of 2 ways:

1: You basically play a "hot or cold" game while holding it. As you get closer to minerals, it would ping more frequently. You could use right click to set it to find separate materials, or you could left click a material while holding the detector and it would search for it. Sure, you might have multiple veins nearby, so it wouldn't give a 100% excellent reading, but isn't that true of real metal detectors?

2: You could use a map-like screen and have a sonar looking thing that tells you what's right underneath you and how deep it is. If you still don't understand what I mean: it would work like a fishfinder. Materials would appear as dots on a screen that shows depth for the block you're standing on.

Also, it could have a "battery" that slowly decreases (like tools' healths) as you walk. You could make a whole new one, or you could use redstone as its fuel (like how coal works for a stove). Like if you right click it would bring up a small screen that just has a one-block storage area and your inventory. You just move redstone to it and it would "recharge" the battery.