One nice trick in building is to include vital information in the description of the room. What I mean by this is rather than making an object that is visible based on the object's long description, put the description of the object in the room description and don't give the object a long description. This will challenge players to more carefully observe the environment they are in. Here's a little sample from an area I wrote called Prrl Zham (note: Prrl Zham, and many other of my areas are available in the ROMLama area library).

Absalom of Yrth

Room:
Name: Barren Fountain
Description:
[You stand] ¹ upon the sacred ground of Prrl Zham, where each Manx goes when his or her ninth life nears its close. Bones of mice, rabbits, birds and other Manx delicacies litter the ground. An ornate fountain lies here dormant. What looks like dried milk crusts the sides.

Object:
Name: fountain
Short Description: a fountain
No Long Description
Extra Description:
Keyword: fountain
The fountain seems to be completely empty. Hang on, there might be something in there....

Note the long description of the object. That's the line that is normally displayed to players. Since it's blank, no object will display. Also note that if players look at or examine the fountain, they'll get some more information on the next logical step: look in fountain. Inside the fountain is a key that they need to progress further in the area.

¹This example shows a good overall technique. However, the usage of you stand in a room description is discouraged. More information on the subject of you can be found on the General Building Tips page. More information on the subject of walking and standing can be found on the Room Specific Tips page.