The northern Pantanal is the headline. It is not the place with the most jaguars. It is the place where you can see them, because they hunt caiman along open river banks in daylight once the floodwaters drop from June. The Transpantaneira, 145 km of raised dirt road with over a hundred wooden bridges, is a wildlife corridor in its own right rather than a transfer.
Around Manaus the Encontro das Águas shows the black, acidic Rio Negro and the pale, sediment-heavy Solimões running side by side without mixing. Basing on the Negro means dramatically fewer mosquitoes than a whitewater river. Inland in Bahia, Chapada Diamantina is a quartzite tableland with a 340 m waterfall you look down into and a cave lake that lights an implausible blue within a specific seasonal window. Fernando de Noronha, 350 km offshore, holds visibility of 30–50 m and a hard cap on visitor numbers.
Country-level constraints that decide whether a date is possible.