Porto Velho's sights are modest but specific. The railway complex and the Three Water Boxes are the obvious historic anchors, while the riverfront gives the city its best everyday scene.
Plan a simple half-day loop in the centre: railway, water towers, cathedral, cultural market, then the Madeira waterfront near sunset. If you have longer, use Porto Velho as a base for onward river travel or trips deeper into Rondônia.