The "subject" that ourselves references is implied in the sentence.
We could write the idea this way
It's also a time for us to relax our mind and discover ourselves when we stroll around unplanned and without any specific purpose.
We can sometimes leave out parts of a sentence that are "needed", because our listener can infer or understand that those parts are "there".
The rules that describe when and how we can do this are complex. The relevant grammar topics include "linguistic ellipsis" and "substitution".
As I was reminded in the Language Overflow chatroom, these pronouns are not always used reflexively. Consider She did it herself. Notice that she didn't do anything to herself. Some linguists call these emphatic pronouns.