Parintins is a very small island, with lots of small shops and eateries. Over the years, there has also been some degree of commercialization. You can easily find internet cafes. The tallest building on the island is clearly the local cathedral.
The best thing to do during the day time is to walk around. Most of the visitors prefer to sit down, get drunk and prepare for the evening. Feel free to indulge in the local soup, called tacaca .. and drink the capetas and caipirinhas. Capeta is a cocktail made with guarana, scotch and cachaca. caipirinha is made with cachaca, sugar and lime. Both drinks are very tasty.
You can also walk around to the part of the island where they make the floats. You can also indulge in face-painting to display you loyalty to either side. Dont expect too many people to speak english .... but everyone is very very friendly.
Every once in a while you will also come across random groups of people performing the boi-dance ... a kind of step dance with very simple movements .... you will get a hang of it in less than 20 minutes.... and its a cool feeling to do it with the locals.
One of the most interesting things to do in Parintins is to visit the "galpão" of the festival teams. The "galpão" is something like a warehouse where all material is built and stored during the year .There you can see the craftsmen working on the allegorys they will present in the Arena.