Rio de Janeiro is one of the best places on earth, maybe even the best :) The city has its special vibe, a contrast of beutiful places and dangerous slums. The samba carnival is a spectacle with everything that is happening around it. The whole city is basically a one big party and it is something worth seeing! Enjoy the great beaches of Cobacabana and Ipanema, take it easy and drink some Caipirinha.. preferably many! Get a guitar (Di Giorgio is good) and go to the beach during the evening and play "The Girl From Ipanema" to really get to the right mood.
Although Rio is pretty close to a paradise on earth, you should be careful. There is a very big favela (slum) located next to Cobacabana and there are lots of robberies and other unpleasant stuff going on in the evenings when it's dark. I got a taste of this having a 20cm knife on my throat but that taught me to be more careful during the night. This can unfortunately happen almost anywhere nowadays.
By the way.. when you have taken a little too much Caipirinha last evening and you go to the supermarket for a bottle of water, you will face a tough choise.. A bottle of booze is cheaper than a bottle of water ;)