As a way station en route to elsewhere and with the surrounding forest pretty degraded close to town, Porto Velho has little to offer in the way of activities – short river excursions are the most you can hope for.
The broad Rio Madeira forms the western boundary of Porto Velho. With headwaters in the Bolivian Andes, it's 3200km long and has a peak average flow of 1.4 billion liters a minute – it's the sixth-largest river in the world. It enters the Amazon River 150km downstream from Manaus.
At 5:30pm on weekdays, and hourly from 8am to 6pm on weekends, riverboats make 50-minute cruises along the Rio Madeira from the dock in front of the Madeira–Mamoré train station (R$18 per person). While not exactly thrilling, this is a reasonable way to idle away an hour or so – with luck you’ll see a few pink dolphins. You can buy snacks and drinks on board.