On several occasions I've been lucky enough to have visited Marajó Island, which is, for me, one of the most beautiful of nothern Brazil's destinations. At the mouth of the Amazon River, Marajó Island is known for its fine, white, sandy beaches. If you stay in its main town, Soure, you'll be within easy reach of several that are sunny and quiet.
I also suggest you take the time to visit one of the island's 'fazendas', large plantations founded during colonial times. During one of my stays on Marajó, I managed to see Fazenda Bom Jesus and Fazenda do Carmo. Both have delicious food on offer and you can explore the local wildlife by foot, boat, horseback or even by riding a buffalo! Photographers amongst you will love the swampy landscape, as well as the brightly red feathered guaras.
During a boat trip on the iguarapés (mangrove-like tributaries), I spotted swimming sloths, capybaras scuttling along the banks and even caimans on a nighttime hunt.