Unlock Secret Events:
In Trial Mode get at least a bronze medal in each event to unlock the Triple Jump and the 100m Breaststroke.
Get at least a Silver medal in the Triple Jump and 100m Breaststroke to unlock the Vault.

Different People:
When you enter your name, enter it as L.A to get a silver metallic guy. Enter your name as Roma to get a bronze guy. (note: This only works in Trial Mode)

Gold Competitor (Works only in Trial Mode):
When you enter your name, put "Helsinki" or "Sydney" or "Montreal" or "Moscow"

White Metallic Competitor (Works only in Trial Mode):
When you enter your name, put "Munich".

Green Metallic Competitor (Works only in Trial Mode):
When you enter your name, put "Mexico".

Red Metallic Competitor (Works only in Trial Mode):
When you enter your name, put "Tokyo" .

Blue Metallic Competitor (Works only in Trial Mode):
When you enter your name, put "L.A" in as your name for any country.

Yellow Metallic Competitor:
When you enter your name, put "Atlanta" in as your name for any country.

Bronze Competitor (Works only in Trial Mode):
When you enter your name, put "Roma".

Unlock Trap Shooting:
Get a gold medal in every event.

Thanks to Revolution readers Adam, Raul is 4 u, James Markey and Psycho_Qwyx!