One of the sights in Belem is the Monument to the Discoveries. It lies a short walk from the Belem Tower and the Jeronimos Monastery. The moment was built as it name suggest to celebrate the Portuguese Age of Discovery during the 15th and 16th Centuries.
The monument is quite large and you can even pay to go to the very top of it. I skipped this as I am sure it has a similar view to the Belem Tower. You might get a better view of the monastery if you do climb it.