Read, swim, sun bathe, work out, listen to music, watch movies, go to shows, eat, visit with other passengers, shop, watch the ocean, ... etc..

I've done a couple cruises with quite a few "at sea" days. The cruise lines seem to work extra hard at amusing people then. Some educational opportunities with university professors, crafts, contests, tours of the inner workings of the ships, etc..

Worked for me. Very well.

Romelle

PS - Depending upon your country, getting Brazil visas can be expensive, time consuming, and a general pain.