How about "Buffet Fillet"? You are offering what most of us gravitate towards if/when a whole spread is at hand. And Nozlee is right to add the "drinks, small plates (although I am beginning to have an aversion to this trend and name), and desserts will be served". Or how about Savories and Sweets?

We call this kind of party, "drinks and snacks". I like the other answers though.

Everything but Dinner is great!

"Everything but Dinner" would be a funny name for a party. Or just call it a cocktail party and specify that drinks, small plates, and dessert will be served!