Sao Paolo - ???? - Rio

But would also like a nice beach town to relax in but where there are also plenty of young people hanging out, nice bars etc for the evening!

Then you should have come before Carnaval. Most places, particularly those harder to get to like Boipeba or Barra Grande, are going to be pretty quiet, maybe a few closer to the major cities picking up a tiny bit on a weekend. There's always a place to drink some beer and maybe dance a little forro, but it's a myth that the party-all-night of the run up to Carnaval is a year round thing in Brazil.

Most beach towns will be fairly quiet after Carnaval. But that doesn't mean they aren't good.

You could fly down to Florianopolis - beaches there are amazing for 5 days. Then fly up to Rio for the remainder of your trip. It should still be fairly lively but more quiet than the summer period.

Remember, 4 days on IG - you'll have to walk or take a boat to most of the good beaches.

You could also visit Trindade - its very quiet and not lively at all. Its 40 minutes from Paraty (which you pass to get to for IG). There are lots of small towns along the Costa Verde that you could visit.

Also, you could do C) but go to Morro de Sao Paulo instead as the transfer between Savlador and Morro isnt as large as Boipeba/Grande....

Your best plan is, if necessary, stay one night in Sao Paulo, and then head out from there on a local flight. If you only have 2 weeks, Sao Paulo is not a place where you should be spending any time. It is a fascinating complicated city, but it's definitely not setup for tourists to explore, and having a worthwhile experience there would take more than 3 days.

You could have a great trip exploring some of the other places you mentioned, like Isla Grande-Rio-Salvador, or Iguazzu-Rio-Isla Grande, or you could visit Isla Grande and Rio, then a short trip to the Minas gerais state, where you can visit fabulous colonial towns including Ouro Preto.

Or you could do a bus tour SP to Rio, stopping in coastal places along the way, like Ubatuba, Paraty and Isla Grande on the way.

Actually if you want some amazing food - Bahian or Minas food is amazing!

Most beach towns will be fairly quiet after Carnaval. But that doesn't mean they aren't good.

You could fly down to Florianopolis - beaches there are amazing for 5 days. Then fly up to Rio for the remainder of your trip. It should still be fairly lively but more quiet than the summer period.

Remember, 4 days on IG - you'll have to walk or take a boat to most of the good beaches.

You could also visit Trindade - its very quiet and not lively at all. Its 40 minutes from Paraty (which you pass to get to for IG). There are lots of small towns along the Costa Verde that you could visit.

Also, you could do C) but go to Morro de Sao Paulo instead as the transfer between Savlador and Morro isnt as large as Boipeba/Grande....

Thanks Trent, Florianopolis is looking like a good option for some beach chillout time.
Just trying to get more of a feel for the place:
what would the atmosphere be like at that time of year (end of March)?
Is there decent restaurants and bars?
Where on the island is best to stay? There seems to be a concentration of airbnb accommodation in the north of the island and also the south east.

Thanks for the help and to all the others who replied, can't wait for Brazil!

During summer its packed. Traffic is a nightmare on that island. However, end of march (I have only been in Jan) would be fairly relaxing and less hectic. Temp is relatively similar (a bit cooler than summer but not much).

Lagoa da Conceicao has some good restaurants. There were some ok ones in Barra da Lagoa. Its a big island, so it really depends where you stay.

After Ressaca, Morro do São Paulo is very quiet. The restaurants on the beach have hardly any patrons and there is maybe one sparsely attended music event on a Saturday night. (We prefer it that way sometimes, the better to enjoy the local atmosphere.) Floripa is much the same after Carnaval. As are most other beach destinations that might be jumping during high season. The beach part is OK, if good weather holds; the party part not so much. Depend on your own group for fun/party, and curb your unrealistic expectations of wild parties and hookups.

Thanks for managing my expectations...

Floripa has the advantage that it's a fairly big city and March is still a good time to go. The jetset parties in Jureré will be over, and your chances of bumping into Neymar or Medina are gone. But there's still plenty happening on the beaches, and the weekend nightlife never goes away (and if the Floripa nightlife isn't satisfying you, you can head over to Camboriú where there's bound to still be hordes and hordes of Argentineans keeping the party going in March).