Others will suggest the best rooms. We always request to be in building 97, but I understand that they are not the new ones.
There are lots of different activities, we don't have kids so I will let others answer.
I wouldn't suggest walking to Sea World. While it is just up I Drive, it is quite a ways to the actual entrance gates. It would be a long walk, over a mile (maybe more) to get to the parking area and gates. I don't think you can enter at the corner of I Drive and Central Florida Parkway.