There are loads of hidden details in both parks, street front windows in the studios in particular, and the entire Port of Entry area in Islands of Adventure, but other areas like Marvel, Potter, Toon Lagoon, and Jurassic Park all have neat hidden references and things to find, these parks are a gold mine for a scavenger type hunt.
A few off the top of my head, the love birds, Cat Balcony, escaped prisoner, "mini-monestary", fire brigade and more in IOA port of entry. Hollywood Blvd shop windows that have cool teddy bears and doll houses, outfits, and more.
Piano that plays Popeye's theme song. word balloons in Toon Lagoon. Amber mine caves in Camp Jurassic, Horton's Egg, Once-ler's House, Dr. Suess himself, and Zax By Pass in Suess, Dragon Eggs in Harry Potter, donut on a reporters desk (marvel), Island medallions on the front gate of Islands.