Here are a few suggestions for interesting, fun or cool looking Chao.
To make the Cockroach Chao you see in Challenge Races, take an Onyx egg, evolve to be Dark, and give dragons until he has the antennae. If you don't have Sonic Advance you can use a Shiny Black instead.
Breed an Invisible Chao (try Shiny Lime Green, Yellow or Grey with a Metallic). Give him a half-fish for the flame ball above his head, and then put a white or shiny white eggshell on his head.
Breed a Transparent Chao (any Shiny except for Shiny Lime Green, Yellow or Grey, with a Metallic). Evolve him to be Hero to get a halo above his head, and/or give him a bat to remove his legs.
Breed a Duotone White Chao and evolve him to Dark Run. He will be black but with white streaks on his body that are similar to bones. Now put a skull on his head (dig with Knuckles or Rouge to find skulls in the garden).
To make a Chao that looks like the Legend of Zelda character, get a Pure or Duotone Green Chao and evolve him to Neutral Swim/Power. The head sweeps back like Link's hat, and the Chao has pointed ears that stick out.
Get a Metallic Blue (sapphire) Chao egg and evolve him to Dark Run. Then give yellow items to evolve him to Dark Run/Swim. This gives a head shape similar to Metal Sonic. Finally, if you want, you can make him win the Topaz race to get a yellow jewel on his chest.
Slightly lame, but good for a laugh: get a Pure Green Chao egg with angry eyes and the v or ^ mouth and evolve him to Neutral Normal. Give him dragons so that he gets green antennae.
Reminds you of the character Reala from NiGHTS: Into Dreams! Breed a Duotone Red. You can then either evolve him to Neutral Fly for the closest head shape or Dark Fly for blue eyes. Either way, keep giving purple items until he becomes Fly/Fly.
Get a Silver Chao egg and give green items until he evolves to Dark Run. Continue to give green items until he evolves to Dark Run/Run.
Get a Gold Chao egg and give green items until he evolves to Neutral Run. Then give purple items so that his spikes stand up. OR: Raise a Gold Chao egg to Dark Run/Run (Shadow) for the best spikes. OR: Raise a Gold Chao egg to Neutral Run/Run (in my opinion this looks least like Super Sonic, though).
Slightly lame, but good for a laugh: get a Pure Yellow Chao egg with angry eyes and the v or ^ mouth, and give him green items until he evolves to Dark Run. Then continue to give him green items to evolve him Dark Run/Run. His spikes will stick up like hair.