Was were. Past Simple
What place is it? you/park/yesterday?
It's a park. Were you in the park yesterday? Yes, I was/No, I wasn't.
What place is it? you/playground/yesterday?
It's a playground. Were you in the playground yesterday? Yes, I was/No, I wasn't.
What place is it? you/beach/yesterday?
It's a beach. Were you on the beach yesterday? Yes, I was/No, I wasn't.