Paint it, lower the ceiling to cover it, build a 3 sided wooden beam or sheet rock enclosure around it. Use your imagination or that of a contractor.
A ceiling made out of dinosaur duct tape!
To hide pipes, wires or duct work.
Make a bulkhead or install a drop ceiling
the steps and the things you'll need will be at this site --> http://www.wikihow.com/Make-a-Duct-Tape-Book-Cover
"Cover her mouth with DUCT-tape, so she doesn't scream."
When you say "exhaust vents in your ceiling" I assume you are referriong to the heating and AC vents inside your house and not the roof vent on the outside used to vent the attic. It is most likely that the duct work was ran in the ceiling for ease of installtion or the house may be built on a slab and they did not want to run the duct work through the slab.
Dirty ductwork, probably a result of infrequent filter changes.
Before you make a decision if you should get your air ducts cleaned, you require knowing a little about how they work and why they collect dirt. You can simply tell the difference between return and supply ducting by going down to your basement and inspecting the sheet metal ducting typically found hanging from the floor joists on the ceiling. The return duct is typically a large rectangle that is attached to the bottom of your heating system, while the supply duct comes out the top of the furnace. The supply duct is usually made up of a large "trunk" duct on the ceiling, along with smaller metal ducts that branch off into each heated room. You'll find them often covered by a metal grate on the floor or the wall.
you want a book first to use as a mold sort of deal. Get tin foil, smooth it and wrap it around the book as you would like to, and cover with duct tape. Voila! A duct tape book cover
NO it is dangerous.
A false ceiling can lower energy bills, make a room quieter, cover up an old ugly ceiling, and make for easier access to plumbing, heating, and electrical equipment. Lighting fixtures are sometimes recessed within this type of ceiling. You'd use a suspended or "false" ceiling to hide plumbing, wires, or duct work in a way that still allowed access without having to tear down and then repair a "hard" ceiling.
Remove the vent cover and cut a piece of sheetrock 1/2 inch smaller on length and width than the opening. attach the sheetrock and use sheetrock mud to fill in the gaps. let dry and a second layer of mud may be needed to get it seamless. then repaint.