Reading aloud is another good way to help her discover that learning is fun. Go to the library and pick out several beautiful picture books. Read and discuss what you like or dislike about them. If your daughter finds a particular interest -- like rainbows -- pick out a book on that subject. Be careful not to "teach" that subject -- just share the knowledge together.
I think she is a bit young to enter the testing arena. Unless you suspect a problem, there is really no reason to have a child tested at such a young age. If she's progressing nicely -- and it sounds like she is -- just let her be herself. You know she's especially bright; you don't need a test to prove it. Enjoy her, and keep up the good work!