Paint shapes and vistas on very very large pieces of paper and tack these panels of your art on the ceiling tiles. Or blow up on a commercial copier (think Kinko's) something famous....like the ceiling of the Cistern Chapel. And tack these up too. OR just tack up full pages from various magazines that you find attractive. I would most likely use the covers of the Magazines because it would be heavier stock. Visit a thrift store to gather materials for this project. I did like the other poster's suggestion of a parachute sheet but I did this once and dust, bugs and dirt just stuck to the sheet like glue.

you could staple sheets up or hang a parachute from the centre

Just moved into and apartment and my room has the ugly tile - like ceiling that is found in offices. Without plastering over the tiles how can I hide or decorate the ceiling so it's not so ugly?