Summer is extremely hot and humid. Winter is pretty mild, though occasional cool days do happen, so a few sweaters or cardigans are nice to have.
There is also a lot of rain, so bags that are waterproof or have a waterproof cover are ideal. Bring a lightweight rain jacket.
Pack good shoes, as you walk a lot in this mission. Also good socks.
Elders: Because of the heat and humidity, lightweight, quick drying golf pants are ideal.
Sisters: lightweight and loose flowy dresses.
A portable rechargeable fan for nights.
The children love little stickers or small gifts that missionaries can hand out.
A filtered water bottle or an insulated water bottle to fill up at home is nice.
Bring small towels or handkerchiefs to wipe down during those hot days.
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Most parents send their Elders with at least 1 suit (most countries in South America only wear suits to church and meetings) and around 8–10 golf pants. Families are sending more shirts because of heat and humidity — 12–14 is the average. Note for Sisters serving in Brazil: feminine hygiene products are expensive there and tampons are often considered taboo. Be prepared to send a good supply with your Sister Missionary.
Serve shirts have a ventilation panel in the side of the shirt making them noticeably more breathable. Missionaries serving in hot or humid places absolutely RAVE about this feature. The most popular shirt for hot-climate missions, for sure.