Brazilians & South America lovers, are you there?

This December I'll be backpacking from North to East Brazil for a month. My current itinerary is based on information I've looked up and some advice from a friend in Rio where I'll be staying at the end of the trip.

Do you have anything to recommend, any must-see's or don'ts, tips & tricks for local transport, nice hostels or friendly poussada's, ... for Sao Luis, Barreirinhas (Lencois NP), Atins, Parnaiba, Jeri, Fortaleza then Salvador's surroundings?

Don't really need info on Rio though as I'll have a local host & guide, but I'll have a whole week to meet with you if you're in the area begin of January.

Looking forward to reading you ... thanks a million !