rc89pl:
I'm sorry but I have to disagree with some of the above suggestions, based on what you have said (23yo, traveling w/ gf, first time in Rio):
- Floresta da Tijuca (Tijuca Rainforest): nice place, but mostly empty and sedate, and you're going to Iguaçu Falls after Rio! Don't waste your time there.
- Niterói: this is the same as recommending that someone goes to New Jersey on his first trip to NYC. C'mon, if you come back to Rio 20 times, then it's time to go to Niteroi (by the way, Itacoatiara is a nice beach, but people who goes there go to get away from the hordes of people, sometimes chaotic, at Ipanema and Copacabana). Don't do it if you're 23yo and it's your first time in Rio . . .
- Petrópolis: a nice town, but very sedate. Forget about it until you're at least 50yo
- Paquetá: nothing positive to say about this one - very, very sedate, the beaches are inappropriate to bathing due to pollution, not much to do there.
- Prainha / Joá / Parati / Ilha Grande / Búzios: these are nice places, but they're secondary destinations; if this is your first time in Rio, keep yourself at Ipanema / Leblon / Copacabana
The positives:
- Lapa at night, preferably Friday or Saturday. Very good program.
- The site at ipanema.com is a good starting point to find other things to do.
Other suggestions:
- The "Lagoa": if, as appears, you like to stroll around, go for a walk along the margins of Lagoa Rodrigo de Freitas (besides Ipanema and Leblon). It's 8Km/5miles, scenic and relaxant, has some kiosks with food and beverages along the way. Could take a half-day if done leisurely or 2h if walking briskly, your choice.
- Arpoador Rock (do a search on YouTube, "Pedra do Arpoador"): Climb it (a very, very easy climb). If you could go there on a sunny day, go in the afternoon and stay for the sunset. There is no money in the world to pay for it (specially being w/ a gf), and it's free. After the sunset, there are the Astor and/or the Fellice bar/restaurants nearby.
- Corcovado and Pão de Açúcar: Both are fine, but I much prefer Pão de Açúcar. The hassle factor to go to Corcovado is high, Pão de Açúcar is much easier. Personally, I also think the views from Pão de Açúcar are better. Near the teleferic that goes to Pão de Açúcar, there is the "Pista Cláudio Coutinho" a small, very scenic trail around the base of Morro da Urca. And, if you like churracarias, there is Fogo de Chão nearby, probably the best one in Rio (this is controvertial, as Porcão and Marius are also very good).
The few days you allocated to Rio won't suffice to do even the basics, don't lose time with secondary activities.