I haven't been to Universal Studios Hollywood in 5 years, and it's still such a great park! If you're only going for 1 day, I would highly recommend getting the Express Pass so that you can go on every ride & see all the shows.
If you only have 1 day, I would recommend starting at the Lower Lot with the Transformers, Jurassic Park, and The Mummy ride and riding those first in the morning since there are less people. To be honest, after riding those 3 rides in a row, I had a little bit of motion sickness.
Something that I thought was pretty cool was that the Studio Tour at Universal Studios Hollywood has the only behind the scenes tour out of all the Universal parks (Tokyo and Orlando). The WaterWorld show was really cool to watch, as well as the Animal Actors show. I absolutely loved the new Secret Life of Pets ride, but it only takes 2 people per car so it'll be quite a wait.
In terms of food, I read online that the Three Broomsticks had the best food of the park, and I have to say that it was pretty good for amusement park food. I also got a churro and a pretzel, and both were pretty tasty. After the park closed, we got food at Citywalk, and even though it's overpriced (Taco Bell there was 2x the price), it's definitely more for convenience. One of my go-tos is Johnny Rockets, and we also tried Bubba Gump which was good, but there might be a bit of a wait (we waited 35 minutes).
We didn't really go shopping at any of the stores, since most of the merchandise is also available at the Citywalk stores (which are open later than the park).
I would highly recommend this park to anyone who enjoys movies, thrill-seeking rides, and of course Harry Potter world. Also, Super Nintendo World will be opening here, which is super exciting!! I can't wait to come back once it opens :)