My middle school students love having fiestas but I make them work for it. This page contains 26 different foods and drinks authentic to the Hispanic/Latino world, a note to the parent, and a sign-up sheet.
Each student must choose a food or drink and then buy it or cook it for the class to sample. Each time I've done this it's been an amazing success and the kids constantly bug me to do it again. There are also things that the students can buy, like Horchata, in case their parents don't want them cooking or if they can't afford anything they can sign up to be helpers.
To make the atmosphere less party like, you can always have the students present what they made and describe the process they went through to cook it. You can even turn it into a webquest!