Ueno Park has paid facilities like Iga Ueno Castle and the Ninja Yashiki (Ninja House), but the park itself is free to enter.
The building of the Iga Ueno Meisan Kyodo Kumiai (Iga Ueno Specialty Products Cooperative) is an old building near the Haiseiden (Hall of Haiku Saints), and it might take a little courage to go inside. I didn't go in, but it seems they sell Iga products.
Many wooden benches and seats are set up around there.