I have about 30 different objects and want them automatically scatter (without pattern, chaotically) in a given rectangle (or at least on page). How can I do it?

There are three approaches to this:

1) Use the Edit > Clone > Create Tiled Clones dialog. This can create randomly arranged clones of an object, but is no use if you want to rearrange existing objects.
2) Use the Tweak Tool . This lets you produce a more "directed" scattering, as objects only move when the mouse is over them. But equally it's not so good for large changes of position.
3) Use the Object > Align and Distribute dialog. This has a button for "Randomise centres in both directions", and an adjacent button to "Unclump" objects, if you don't want them to overlap.

These tools have all been covered in my tutorial series (parts 33-36, 22 and 39, respectively): viewtopic.php?f=6&t=11981