We did Universal and IoA for the first time last December in one day. It was a gloomy Thursday so that probably helped crowd-wise too. We got to Universal right at opening. We went to the right and did E.T. first, then the Simpsons, then Men in Black, then Jaws (R.I.P.). My brother and fiance did all the more intense rides by themselves and I kind of just wandered around with my parents. We're not show people so no shows were seen during our time there.
My brother and fiance rode the Mummy and Rip Ride Rocket. By this time with an average wait time of 10-15 minutes for each ride, it was almost noon. We walked over to IoA as we had a reservation at Mythos. We walked through the Seuss area and stopped at Mythos to eat lunch. After lunch (approx. 1:30), we went back to Dr. Seuss area and did the Trolley in the sky (longest wait of the day, 40 minutes, and sooo not worth it) and Cat in the Hat. We skipped Harry Potter as it still looked crowded and rode the Jurassic Park river ride. We wandered around the Toon area and rode the two water rides there (no wait 'cause it was cold and cloudy). My brother and fiance did Hulk and Spiderman as I consumed a whole bag of cotton candy

. We made our way back around to Harry Potter at approx. 5 p.m. (I believe they closed at 6). The place was like a ghost town. Forbidden Journey was a walk on (but the shops were still packed). We spent plenty of time there, got a butter beer, and were headed towards the exit a few minutes before 6.
It was a busy and fun day. We ended up going back later in the trip (we hadn't planned to, but we had 3 day passes that don't expire and we decided to go back on one of our unplanned days).
I'm sure this plan would be harder to follow if it was very busy and/or you couldn't stand getting to the Harry Potter area so late in the day

. We never felt rushed and felt like we got to do most of what we wanted in that day.