I had fun with a beholder floating right behind a pit trap. Good guys saw the monster, charged, and *plummet*
Should work with any hovering creature.
Long cavern hallway. Goblins hiding behind a wall at the end, shooting through arrow slits. The wall gives them cover making them almost impossible to hit, and even protecting them from area effect spells. The length of the hallway stops the adventurers from even seeing the goblins in the dark at the other end (The adventurer's side of course is lit - if not, shoot a couple of arrows with continual flame or flaming rags). Length of hallway also makes sure the goblins get a few rounds off before the good guys can close - and when the do, they can't get thru the wall (at least not immediately, giving the goblins time to escape)
A variation on your silent image - I had a permanent image of a wall covering one side of a T intersection. Adventurers came and turned the corner. My ambushers came through the wall and get to attack the old bearded guy in the back
PS: can you put a link to the handbook in the first post? Please and thank you.
PPS: and a link in your handbook to this discussion thread.