When traveling from the Tom Jobim international airport to Centro [1] or Zona Sul you’ll get your first sight of Rio [2]’s sprawling Zona Norte district. For the most part, there is no reason to see more of it than the glimpse afforded from your bus or taxi window. The area is largely residential with working class and poor neighborhoods alongside vast favelas [3]. Poverty and drug-related violence has made large pockets extremely unsafe.
The exception is the Quinta da Boa Vista [4] (which is fairly close to Centro and easily accessible by Metrô), a vast park with some interesting sights, among them the Museu Nacional [4]. If you have some extra time to kill while in Rio, you can combine its attractions into a day trip.
Links:
[1] http://www.moon.com/destinations/brazil/rio-de-janeiro/rio-de-janeiro-city/sights/centro
[2] http://www.moon.com/destinations/brazil/rio-de-janeiro/rio-de-janeiro-city
[3] http://www.moon.com/destinations/brazil/rio-de-janeiro/favelas
[4] http://www.moon.com/destinations/brazil/rio-de-janeiro/rio-de-janeiro-city/sights/zona-norte/quinta-da-boa-vista-and-museu-nacional