I much prefer shirts to polos and tshirts. Cotton or linen shirts are as light as tshirts, but can look much better. If it's warm, you can fold the sleeves to the elbow. Or use sleeveless shirts - some people think they're a little geeky, but not to me-as long as one is careful with the print. I use sleeveless shirts a lot in DC's hot weather.
Jeans with a shirt are fine for a young person, but if you want something lighter, linen, light cottons or light wools are even better. Since you live in Brazil, you can also use nice colors for the pants, like pale green, light blue, pink - a white shirt with pale green pants and light colored loafers seems perfect.
For even classier looks, in Italy we use a lot suits made of "fresco lana", ie very light wool. But these tend to be very expensive, because the fabric needs to be very good to remain light and fresh.