Manaus sits at the junction of two big rivers in northern Brazil, the Rio Negro and the Rio Solimões, which combine to become the mighty Amazon.
Ponta Negra beach, about eight miles from downtown, is arguably the most popular local beach, while Praia da Lua beach just beyond it is a bit more secluded and quiet. You can reach Ponta Negra by bus or taxi; to get to Praia da Lua, you will need to take another bus to the marina, from which you can catch a boat to this more remote beach. Plan to get back to Ponta Negra by the evening, since the area is also famous for its night life.
To get a deeper understanding of the Amazonas region, you can spend a bit of time in the museums of Manaus. There are a couple of museums in Manaus dedicated to the region's indigenous populations. Spend the remainder of your last day scouring the busy shopping streets and Mercado Municipal in Manaus, picking up any additional souvenirs you want to bring home.