The "Island of Fear" — Ilha do Medo — is a small island in the bay of São Marcos, accessible by boat from the city's waterfront. In the colonial period, it was used to quarantine ships arriving from plague-affected ports; today it's uninhabited and largely forested, with good birdwatching and views back towards the city skyline.
Boat trips to the island can be arranged from the Cais do Porto (the old port waterfront) and typically combine the island visit with a tour of the bay. The contrast between the tranquility of the island and the views of the city and the industrial port makes for an interesting afternoon. Ask at the waterfront for current boat operators and prices.