Yes, definitely plan to spend both days. The shows will take up pretty decent chunks of time, so going a second day will allow you to enjoy shows, rides, exhibits and interactions a lot better. Get a QQ pass for both days, as they are only one time through each attraction. Kraken usually doesn't have too much of a line because it has excellent capacity, but Manta can develop a decent one even on not-so-busy days. They will be back at full capacity by then (right now one out of the three trains is out of service for annual maintenance) but it's still a lot longer process to dispatch those trains than Kraken's.
As for what else to do - try to get to all the shows. The only one I haven't seen so far is Clyde and Seamore, but One Ocean, Pets Ahoy and Blue Horizon are all amazing. Antarctica: Empire of the Penguin is an ok ride but the penguin exhibit at the end is what makes it really worth it as you are really close to the penguins. Buy some food to feed the sting rays, it's really cool to feel them eating from your hand. If you don't want to buy food, stand on the side of the tank next to where the food is sold - they're used to being fed there and are more likely to swim close so you can pet them. Don't miss the aquarium in the gift shop at the exit of Journey to Atlantis. It's the most overlooked aquarium so it's usually pretty empty, and there are tanks above, below and around you, which is really unique.
They recently closed off part of the non-show viewing of the whales due to construction of Mako. The coaster won't be completed yet in April so expect some other area and possibly exhibit closures as they work on it.