Here's my thoughts from our trip with our 2 year old (last week). We also had our almost 6 year old son with us.
The 2 year old couldn't ride much in comparison to my older son, but it didn't seem to bother him at all. The rider swap at Universal is incredibly awesome. Not sure how much you know about it. But, basically, we all were able to get in line together. Then, while my husband and son rode the ride, my youngest and I sat in an air conditioned room and watched a movie. Then, when they got off the ride, my oldest and I could ride again, immediately. This was really important because my youngest knows a lot of about Harry Potter and Transformers, so he was so happy to walk through the castle, gringotts, and the transformers line.
He was able to ride: Hogwarts Express, One Fish Two Fish, Dr. Seuss Carousel, Simpsons ride that's like Dumbo (can't think of the name off-hand), Storm Force Accelerator (like Teacups), Despicable Me (sat in a non-moving seat because of height) and Shrek 4-D.
He also really enjoyed spending time at the Curious George and Barney Play Areas. Seeing the Barney and Dr. Seuss Shows were also highlights. And, meeting The Simpsons (other characters didn't garner the excitement that Bart and Homer did).
My son is pretty easy going and just enjoyed walking/running around and even being in his stroller. I was surprised he also really enjoyed Butter Beer and starting asking for it by the second day. And, of course he had to have a wand like his brother.
Snacks were key to survival for us as surviving breakdowns (usually from us going too long without a meal).
Sorry to be so long-winded. But, I think it's totally do-able. I loved seeing him get excited about the littlest things and just having an all around great time. Now, everyday he asks to watch E.T. and tells everyone he rode the Hulk while we were there. Haha.