What To Wear To A Tea Party In The Park

For my friends 18th birthday she is having a tea party in the park, but i really don't know which outfit to wear? I have a tea dress, but i didn't want to go that smart. Please help. http://www.polyvore.com/cgi/set?id=18598... http://www.polyvore.com/cgi/set?id=18603...