You are going to LOVE Universal Orlando. LOVE it. It's fantastic. When I visit Orlando, Universal is my primary destination every time, above Walt Disney World. The two parks are snuggled right up to each other and VERY easy to walk between. For that reason alone, get a Park-to-Park ticket. It'll let you bounce between the resort's highlights, especially at the end of the second day. You'll be kicking yourself if you go to Islands of Adventure on Tuesday, then Universal Studios on Wednesday and realize you can't run back and re-ride Spider-Man that night.
Also remember: Diagon Alley has two major attractions: Harry Potter and the Escape from Gringotts, and the Hogwarts Express. The Hogwarts Express is a fully-immersive dark ride (so it is NOT a simple train like Kings Island's or the Walt Disney World Railroad) that travels BETWEEN Diagon Alley (at Universal Studios) and Hogsmeade (at Islands of Adventure). You cannot ride the Hogwarts Express attraction unless you have a Park-to-Park ticket. You board in Kings Cross Station at Diagon Alley and get off in Hogsmeade at the other park (or vice versa) so without a Park-to-Park ticket, you will miss that E-ticket, headlining, major ride.
There's nothing I'd tell you to skip, but with time being short, focus on the obvious headlining rides (Forbidden Journey, Gringotts, Revenge of the Mummy, Spider-Man, Transformers, Jurassic Park, Hulk, Dragon Challenge, Men In Black) but be sure not to miss Poseidon's Fury, E.T., and the Horror Make-Up Show.
You'll find that Universal's two parks contain as many headlining attractions as Disney World's four, and doing it in 2 days won't be easy! If you're staying on-property at one of the resort's three deluxe hotels, your room key counts as FREE, UNLIMITED front-of-the-line passes (not good on Gringotts or Forbidden Journey, last I checked). You can also purchase once-per-ride Express Pass Plus online or at the park, with prices fluctuating based on expected crowd levels. It can be pricy based on the time of year, but it will definitely relieve pressure from your vacation and allow you to relax a bit.