Singing was his favorite activity

Can "his favorite activity" be thesubject also?

But what if you turned it around:

His favorite activity was signing.

No. It's a subject complement.

http://www.chompchomp.com/terms/subjectcomplement.htm

Then activity would be the subject.

No, in the given sentence "Singing was his favorite activity," "Singing" is the subject. The verb "was" functions as a linking verb, connecting the subject "Singing" to its complement "his favorite activity." The subject of a sentence is the noun or pronoun that performs the action or that the sentence is about. In this case, "Singing" is the action being performed, making it the subject. "His favorite activity" is a noun phrase that acts as the complement, providing additional information about the subject.