We loved the frozen butter beer and pear cider, but eight of the nine people in our group made some really interesting faces after trying the pumpkin juice. (My husband has a cast-iron stomach.)
The cauldron cake was my favorite treat from our two visits (it's really rich, though. Three of us split one). And we loved the pumpkin tart. The chocolate frog is problematic: a really big fan is going to want to buy one at least once. But the chocolate they use tastes really cheap (kind of a chalky, waxy aftertaste). My son ate part of it, but ended up pitching most out. One thing you don't hear a lot about is the chocolate trifle at the Three Broomsticks -- we really enjoyed it.
On our most recent visit, we tried the Boulangerie at the Studios. I was all sugared out by that point, but my husband and son both gave the cheesecake a thumbs up.