Serious is the opposite of Ridiculous. Ridiculous is a synonym of Dumb. Dumb is a synonym of Silly. Silly is a synonym of Stupid. 
Thinking can be used for Creativity. Thinking can be used for Reason. Reason is related to Logic. 
Serious <--Antonym--> Ridiculous <--Synonym--> Dumb <--Synonym--> Silly <--Synonym--> Stupid 
Creativity <--UsedFor-- Thinking --UsedFor--> Reason <--RelatedTo--> Logic 