We've only been to Hogsmeade (fixing that in June!), but I would make sure to try both the frozen and regular butterbeer (now they have ice cream and hot butterbeer too?); going through the queue at FJ; sitting under the awning in front of 3 broomsticks to watch everyone go by and soak in the ambiance; getting a treat at Honeydukes (the boys loved the cauldron cakes).
I am looking forward to doing the following on this summer's trip: finally doing Ollivander's; seeing the interactive wands; riding the coasters (neither of my boys would ride them in 2012); seeing more streets / scenes in Diagon Alley; eating at the Leaky Cauldron; taking a photo of my boys in the gap in the brick wall coming into Diagon Alley; riding Gringotts.