Best way to visit the other parks?

Both of the Universal parks are chock full of thrill rides, including roller coasters. Unfortunately they're rather slim on interesting non-thrill rides, so I end up spending a lot of time waiting around while friends and family ride stuff.

I can't help you with the transportation questions; we've always had a car when we visited non-Disney parks.

We used Mears Transportation to get to Universal, but it will also take you to Sea World, not sure about Busch Gardens. It was either $18 or $19 a person round trip and, if I remember correctly, they did pickups and drop offs from the resort to the park and vice versa in hour incriments. All you have to do is call the night before, tell them which resort you are at, and they will tell you the available pickup times for your resort and you choose from there. Then your bus driver will tell you what available times the shuttle will be at the park pickup spot to take you back. Once you decide what time you want to go back to the resort, they ask that you call 2 hours in advance to let them know to make sure there is room on the return shuttle. It sounds complicated but it was very easy and convienant. If there are only 2 people in your party, it made the most monetary sense as well, however any more then that and it'd probably just be more cost beneficial to rent a car.

http://www.mearstransportation.com/

Yes, that's an Aristocat.

I've only spent one full day at either of the Universal parks, and most of that time I spent tagging along and waiting around. For our upcoming trip I plan to split off and explore on my own while the others do the thrill rides. I am no Universal expert so I don't remember the names of the themed areas at the Universal parks. At Islands of Adventure, the Dr. Seuss-themed area looked interesting, and the people I travelled with two years ago had no interest in it, so that's one area I plan to explore on my own in April. The Harry Potter area is awesome even if you ride nothing. And you can walk through Hogwarts without riding, so don't miss that. Universal Studios has more non-thrill attractions that I never got to see on our last trip, and I plan to visit them all this year.

As for snacking and chilling, the dining choices at Universal aren't as interesting as those at WDW, especially counter-service. Two years ago I ate at Mythos and Lombard's, both pretty good table-service places. The food is about as good as you'll find at an average WDW table-service restaurant, but the prices are lower so that's an advantage. For counter service, Three Broomsticks looks like fun, but I didn't eat there, I just walked through.

Just a heads up on Busch Gardens. I would seriously doubt there is busses as it is in Tampa, at least an hour away. It's a bit of a drive, but if you are looking for a good combintaiton of thrill rides and other attractions, this may hit the spot. Some good coasters, but you can check out animals too. I know some Disney faithful don't care for it, but I think it has some heart.

A cab to Uni from Epcot resorts area is about $35 one way plus tip.