The dropping of a rock is an interesting idea. What about disintegrating large areas of a ceiling, such that a broad, heavy, flat piece of ceiling is attached to the top by one solid column of stone? The beholder could hide between the fake ceiling and the real ceiling; when PCs were beneath the fake ceiling, the beholder could disintegrate the column, dropping the fake ceiling onto the PCs. Ouch!
Similarly, it could carve out large pits beneath its lair, which had a thin false floor covering them. If facing nasty fighter-types in combat that are immune to its charms and resisting fort saves, it could disintegrate the ground beneath their feet. The pits could be empty holding cells, or they could be filled with carrion crawlers, oozes, or other nasties.
Finally, combine these two: a thin false floor over a trap, and a heavy false ceiling over the false floor. When the PC is standing beneath the false ceiling, disintegrate the column holding it up. The ceiling drops on the PCs and on the false floor, breaking the false floor and sending them plummeting into the pit trap. Hee hee -- think of the fun you'll have describing this to your players!
The best part, of course, will be the rogue who makes her reflex save and jumps clear of the falling ceiling of death -- to find herself alone and looking up at a Beholder staring back down at her.
Daniel
