Line the window sill with pretty colored bottles that will allow the sun to filter thru them thereby creating your own rainbow.

Use a single tension rod and once in place, simply drape pretty placemats over them...or solid colored ones...or bamboo ones...OR old mini blinds that serve no purpose that you can cut and lay over the rod.

Hang dried flowers from a tension rod OR fake ones

Use a vine and hang wreathes from it

Depending on the season, tape or hole punch and hang pretty CARDS such as the ones your son makes, from a TENSION ROD or string of some kind.

Cut off a sheets and PIN it over a TENSION ROD...or a pillow case works nicely too

Find an old quit at a flea market that has seen better days, is dirt cheap, and can be cut up (oh that kills me to say that!) then cut it and hang it...yes, you guessed it...over a tension rod and try being creative by hanging it diagonally, or just on one side of the rod...put a vine over it...or hang a wreath on the "blank" side....

String beads and hang them vertically

wheww....okay...that's all I can think of off the top of my head...