We did one day at Universal last month, and WWOHP was the priority. We got there at 7am (onsite guests), entered UO, and walked straight back to Diagon Alley. We spent up until about 10:30am there. We rode the Gringotts Rode, bought butterbeers, watched the wand show and purchased wands, did several of the interactive wand things, shopped, ate breakfast, watched a puppet show, and shopped some more. We then took the train to IOA. It was much, much more crowded and HOT there! We rode Forbidden Journey and then took a pool break! We came back to IOA at around 4 for dinner and more exploring (still very crowded), took the train back (must ride both ways), went to the Simpsons World, then left. It is totally doable. Diagon Alley was a much more pleasant experience than Hogsmeade.