Q:What would you like to do this weekend?

A: I just want to relax at the weekend. => I just want to relax this weekend.​ 

=> Corrected: The sentence is corrected grammatically. 

-나는 수족관에 갈거야.

I'll go to the aquarium. => I'm going to the aquarium. => Your sentence has a correct grammatical structure, but this has the correct usage/expression.​

-와, 멋지다! 나도 수족관에 가고 싶다.

Wow, cool! I want to go to the aquarium. => Wow, great! I want to go there, too. 

=>Your sentence has a correct grammatical structure, but this has the correct usage/expression.​

-너는 무엇을

What do you => What are going to do? => Corrected: The sentence is corrected grammatically.​​