Here is a list of 'opposite' words:
/:weepy:
Above/Below
Adult/Child
Always/Never
Arrive/Leave
Awake/Asleep
Back/Front
Backwards/Forwards
Before/After
Better/Worse
Big/Small
Black/White
Boring/Exciting
Borrow/Lend
Boy/Girl
Brother/Sister
Clean/Dirty
Clever/Stupid
Coward/Brave
Dead/Alive
Difficult/Easy
Dog/Cat
Dull/Shiny
Energetic/Tired
Fast/Slow
Fat/Thin
Flat/Bumpy
Float/Sink
Formal/Casual
Friend/Foe
Full/Empty
Generous/Selfish
Good/Bad
Hairy/Bald
Happy/Sad
Hard/Soft
Hello/Goodbye
Here/There
High/Low
Hit/Miss
Home/Away
Hot/Cold
Hungry/Satisfied
Interesting/Boring
Left/Right
Light/Dark
Light/Heavy
Loud/Quiet
Man/Woman
Messy/Neat
Nervous/Calm
Nice/Nasty
Night/Day
Old/Young
Open/Closed
Polite/Rude
Question/Answer
Rich/Poor
Right/Wrong
Rough/Smooth
Sharp/Blunt
Son/Daughter
Spring/Autumn
Stop/Go
Summer/Winter
Sun/Moon
Sweet/Bitter
Sweet/Savoury
Sweet/Sour
Tall/Short
Tame/Wild
Thin/Thick
Together/Apart
Top/Bottom
Up/Down
Walk/Run
Wet/Dry
Wide/Narrow
Win/Lose
Yes/No
I also came across this site:
Live and Learn which is an animated opposites 'book'.