Here's stuff I've seen personally. I know other sites that have a complete list, and I'll add a link if I find them again.
Flamingo, picnic basket, garden gnome, white chicken, brown chicken
Fuel refill, brown chicken, black chicken
10 fuel refills, brown chicken, gold chicken
20 fuel refills, carousel, zen garden, farmville float, gold chicken
Also, I think they change the prizes around sometimes, so some of these may be out of date.