Well, I WOULDN'T skip Spiderman. But I might skip Posidons Fury (it was long, sometimes hard to see if you were stuck in the back, some special effects were sort of neat but the story didn't make a lot of sense to me....not my favorite and I might not do it again) Hulk, dueling dragons and that tall drop thing would make me sick so I'd have to skip those.
I enjoyed spending time in the market area around ...what is it?....where they have the fortune tellers, etc....
I'd skip Mythos; sorry folks, I think this restaurant is highly overrated.
For Universal, I'd personally not feel bad if I missed Men In Black next time. Wouldn't be heartbroken if I didn't see the Ghostbuster show (DON'T Flame me Fly Fly. It was a good show, I enjoyed it but IF I HAD TO MISS SOMETHING, this would be missable for me on a second trip...Still think the guy in the cowboy hat looks like the psycic John Edward....) Universal has a lot of good shows, rides, it's hard to say what to skip....
I LOVE the terminator 3D and ET, both very good and must sees. I haven't seen shrek yet or the new ride...Jimmy netron. If I had limited time, I'd focus more on universal studios rather than IOA, but that is just me....