Let's make up a simple example: a room with one entrance, 25-foot high ceiling, empty except for a 12-foot high marble pedestal with a two-foot diameter midnight blue globe sitting on top. The globe gives off a faint glow, so adventurers will see the essential details of the room immediately. As a living being approaches the globe, the glow gets brighter, fading again if the living being moves away (or ceases living.) If someone manages to touch the globe, it reaches its maximum brightness of full daylight while touched.
Now, players may peek in the room, notice the glow, but can't figure out anything special, so they move on. Or they may spend a little time figuring out the way proximity affects brightness; this is a bonus, from the GM's POV, because it forces players to decide how much time to waste, risking a wandering monster encounter. The players may even try to remove the globe and put it to use, which is also a bonus, since it rewards creative play.
The important thing to remember, when placing a room like this, or any kind of room (even an "empty" one,) is that there should always be at least one obvious thing to do, and the GM should not shoot down a player's attempts to do things. The GM judges actions not specified in the rules or encounter notes; the GM should never forbid actions not specified in the rules or notes, merely because they weren't specified. This is bad GMing:
GM: You look in the room and see a glowing, midnight blue sphere on top of a marble pedestal.Always say yes, unless you can make it more interesting. If you have a room of ghosts who seem oblivious to the presence of the adventurers, that doesn't mean that the ghosts aren't useful, even if the notes don't mention anything useful. If they speak to each other, maybe they will say something interesting, a hint about what killed them, perhaps? If they are oblivious to the adventurers now, that doesn't mean that they will remain oblivious; perhaps, while experimenting, the players will suggest something that sounds like it could work. Maybe a cleric can use Turn Undead to command the ghosts to pay attention? (Speak with Dead should work, in any case.) Maybe Invisibility will paradoxically make someone *visible* to a being that can't normally see you?
Player: I climb the pillar to look at it closer.
GM: You can't. The pillar is unclimbable.
Player: What?!? Well, OK, let's form a human pyramid and try to lift the sphere down.
GM: You can't. It's stuck to the pillar.
Player: Well, I break the pillar with my ax.
GM: You can't. It's unbreakable.
This is why I think of these rooms as "Rorschach rooms". They are tests to see what explanation the GM comes up with, or what uses the players can devise.