Islands in July too cold?

Also, I'm not only interested in weather they are cold or warm, but if there is much to do if it is too cold. Is one island better than another if you can't be on the beach?

Its premium time to surf even if it is cold. The swells are always better in winter generally.

There are plenty of trails and hikes to do. Boat trips etc.

Generally, the weather will be in the low 20's Celsius. But water temp can be frigid.

Thanks, sounds like we would be able to keep ourselves busy with hinking, etc. but we won't be surfing so if the water is too cold we might be better off somewhere else. Would the water be warmer up the coast somewhere or is it all generally cold at this time of year?

How far "up the coast"? The water may be warmer, but it's rainy season in much of the northeast in July.
It's great sightseeing weather in Rio, and probably warm enough to sit out on the beach in the afternoon.
Or fly to Belo Horizonte and use it as a base to explore the Colonial mountain towns.

Ilha Grande has some great treks. Paraty is worth a visit. So is Trindade. All within a few hours of Sao Paulo. Floripa is further south and probably too cold. Rio is a great at any time of the year - though given the time frame of July it could be expenisve (due to WC).