因为to visit his uncle作表语，且was前面有did这个助动词，所以不定式符号to可以省略。

The first thing my brother did was brush his teeth.

The first thing John did was visit his uncle.

The second thing he did was visit his friend.

The third thing he did was visit the zoo.

The fourth thing he did was visit his girlfriend.

——出自 English for you