Why not do 和 why don't 意思是一样的,但用法有点差别.why not 后面直接接动词原形.why don't 后面要先接主语（即名词或代词） 才可以接动词原形.举例 why not come ealier?等于why don't you come earlier?
本题中,Why not go to the Great Wall this Saturday?等于Why don‘t we go to the Great Wall this Saturday?