Using Mage Hand to drop an object on an opponent? Oh well that's trivially easy to resolve.
Page 42 of the DMG.
Intelligence based attack vs Reflex, with damage of 1d10+3. Roughly comparable with magic missile, except it takes more trouble to set up. It can be useful for ambushes though, so nobody knows where the shot came from.
A particularly dangerous object might be 2d6+3 (a little better than magic missile), and a jar of acid that shatters upon impact could be 2d10+3 or even as high as 3d8+3.