Antonyms for the word "smash up" include "repair," "improve," and "construct." These words represent the opposite action to smashing, which means to damage or destroy something. Repair suggests fixing a broken object or restoring it to its original condition. Improving means enhancing or making something better, which is the opposite of damaging it. Constructing involves creating something completely new instead of breaking down existing objects. Therefore, the antonyms of "smash up" signify actions that are positive, constructive, and beneficial, emphasizing the importance of preserving and improving rather than damaging and destroying.