Discovery Cove is indeed a lovely day, but waiting a couple of years til they're older and will remember it better is a good idea. Not everyone has to do the dolphin experience; there are different prices for with and without.
Last year we went to Orlando for my grandson's 8th birthday, and of everything there his choice was Islands of Adventure. He loves to do the Discovery Center, scanning the "dinosuar eggs -- but that wouldn't appeal to everyone. The character brunch is fun -- Spiderman and Cat in the Hat, etc. We love the Spiderman ride -- the best one of its kind. You want to head to the Pteranodon flyers first thing when that area opens -- the line for it is prohibitively long most of the day. Like Disney -- you want to be at the park when the gates open. It has a range of roller-coasters for those who do those (I don't). The drop from the Jurassic Park river ride was too much for us, but fine for most who like that sort of thing. I really like the Bluto's barges. Don't bother with Poseidon -- it looks wonderful on the outside, but is really lame. The talking fountain near the stunt show is fun. It's a more thrill oriented park than Disney, but also lots of visually neat stuff. Even with a few negatives for us, it might be our favorite park -- (well, Magic Kingdom will always be my all-time favorite, and I really like Epcot) -- for its mix of things.
I'm sure you can get a cab -- and that has the advantage of a closer drop-off and pick-up. I have trouble imagining being without a car, even staying on property (rental prices usually don't seem too bad in Orlando). Using Disney transport is restrictive, and we always seem to need something from a supermarket, or want to go to someplace non-Disney once or twice.
I don't care for Disney Quest, but expect when DGS is a couple of years older, may try to suffer through it again for him.
