The stadium covers nine-and-a-half acres of land. Would the complete predicate be covers nine-and-a-half acres and the verb be covers?

Right! :-)

In Houston my classmates and I visited the Lyndon B. Johnson Space Center. Would the complete predicate be visited the Lyndon B. Johnson Space Center and the verb visited?

Almost right.

In Houston is part of the predicate because it modifies "visited."

Yes, you are correct. In the sentence "The stadium covers nine-and-a-half acres of land," the complete predicate is "covers nine-and-a-half acres of land."

A predicate is the part of a sentence that contains the verb and provides information about the subject. In this case, the verb is "covers," which is an action verb that expresses what the stadium does. The remaining words, "nine-and-a-half acres of land," provide information about the verb and complete the predicate by specifying what exactly the stadium covers.