Most of Manaus [1]’s sights are located around the port and the oldest part of Centro that spreads up from the Rio Negro to the area surrounding the Teatro Amazonas [2]. It is easy to get around Manaus’s somewhat grungy center by day. However, at night, the neighborhood around Praça da Matriz is pretty dodgy, and the riverfront and dock area is downright dangerous.
A maximum of two full days is really all you need to see most of Manaus’s major attractions before heading out into to the depths of the jungle.
Links:
[1] http://www.moon.com/destinations/brazil/the-amazon/amazonas/manaus
[2] http://www.moon.com/destinations/brazil/the-amazon/amazonas/manaus/sights/teatro-amazonas