A lot has changed in 20 years. You will definitely need a whole day. Sometimes even 2 if you want to take your time and enjoy shows, rides and exhibits at a relaxing pace. You should be fine crowd wise next Sunday. It won't be to bad at all. I would find a park map and show times online in advance. Plan to visit shows, rides and attractions around the shows you want to see. So that way you're not running back and forth all over the park. For example, Dolphin feeding is right next to the dolphin show. So plan to do dolphin feeding before or after you see the dolphin show. I would recommend doing the first feeding of the day at 9:30 and then seeing the 10:00 Dolphin show. The first feeding is always the best because it's the dolphin's breakfast and its the least crowded. Have fun!