Cariocando no Rio is both an app, available at the app Store for iPhone and iPad, and an Instagram feed, with very nice and up-to-date tips about Rio. Behind the scenes is Nicole Casares, a very carioca girl who loves the city and is always out and about. If you want a personalised tour of the city or some different and weird request, she is the one who will be able to help you.
ps: the app is in Portuguese only but it is not that hard to use it.