Saturday Night in Clayton, MO
I would highly suggest Harvest, which is on Big Bend right off of Clayton Road. They have excellent food, impeccable service, and a fantastic wine list. I think they also have summer specials as well.
I haven't lived in St. Louis for a few years now, but every time I go back I make it a point to go there - I dream about the bread pudding with the bourbon vanilla sauce!
Where I recommend you go in Clayton depends on what kind of restaurant/atmosphere you want. Here are some suggestions of the best spots, though. For tapas, head to Barcelona. They have an excellent selection and the food is top notch. It is a fun environment and a great place for some sangria. Try Remy's for a leisurly meal. Get the bread pudding with extra sauce! The food is great and the prices are affordable. I would say the Crossing for a very nice, elegant (and more expensive) meal.