I also second butterbeer and the Floreans ice cream. I prefer frozen butterbeer, especially on a hot day. Of the Floreans ice cream, I found the chocolate chili to be the most unique to any ice cream I've ever had. The first few bites/licks taste like totally normal chocolate and you'll wonder if they gave you the wrong flavor. Then you start feeling a tickle in the back of your throat. Then as you continue eating it you really start to feel it. It's what I image Fred and George Weasley would sell as a trick ice cream flavor!
Don't skip the candy and snacks in Honeydukes. In their confectionary counter they have cauldron cakes which are kind of like cauldron themed cupcakes that are really good. As a note, the chocolate frog isn't really great, but if it's your first time going and your a HP fan it's nice to get it just for the decorative box and wizard card. The pumpkin juice also comes in a really cool bottle which is nice to display when you get home, but the juice just reminds me of apple cider, nothing too special, and honestly kind of a heavy drink on a hot day. I do like the Chocoballs (I think that's what they are called) which is like a chocolate ball shell with strawberry cream filling.
It looks like Universal Studios is also trying to get into the food festival game like Disney. From what I've seen, depending on the time of year, they have some booths and food trucks set up with unique stuff. I think right now they are doing an Olympic themed event with different country food booths. You can probably find a lot of good stuff in there!