I lived in São Paulo for a year and went back to a Brazil twice on my own since, although last time was about 15 years ago (seems like we were all there around the same time!) I LOVE Brazil an£ would move there in a heartbeat.
Some of the places I loved:
Salvador is amazing, although they have 'cleaned' it up since the last time I was there - I understand it is less seedy but has lost a bit of the colour.
Rio is beautiful, but I did feel it was somewhat over-rated, for example Copacabana beach is lovely, but there is a really busy road right next to it!
I absolutely loved Brasilia - it is a strange place, feels like a film set in places! But I found it fascinating and could have happily spent longer there (I was only there for 3 days)
Belem- of all the places I went this was were I felt least safe, although it is a really interesting place
Curitiba - I loved it! When I was there it was a really forward thinking place - recycling bins everywhere, colour coded buses so you could see instantly if the bus was express, suburban, local etc. Libraries (shaped like lighthouses!) all over the place, incredible botanical gardens... it was one of the few places I went back to. Also, if you go there make time to go to Ilha do Mel - just gorgeous.
Florianopolis - amazing beaches!
Iguazu Falls - definitely worth a trip if you can.
Generally I felt safe - I speak Portuguese which I am sure helped get me out of some potentially sticky situations! Be sensible with valuable etc. Take/Buy a cheap plastic watch and phone when you are there. It will help massively that your DH is Brazilian.
You will have an amazing time wherever you go! (I'm very
! 
)