Hi,
going to Rio (and Argentina) for the first time next week. Have an extra day in Rio and a private guide. What would be the most interesting place to take a day trip to? thanks!!

It depends on what you want to see. For the average tourist, I would say Buzios. Niteroi is also a beautiful place and much easier day trip, very close to Rio's Downtown (13km apart):

http://wikitravel.org/en/Niter%C3%B3i

I quite enjoyed a day trip to Petropolis.

Thank you both. I narrowed it down to Niteroi and Petropolis. Any tie breaker votes?

Both are great, but very different day trips. Petropolis is the imperial city, surrounded by forested hills. It has a great imperial museum. You'll need 1h30 for each leg to go there coming from Rio's main bus terminal (Rodoviaria Novo Rio).

Niteroi is a seaside city on the other side of the Baía de Guanabara (Guanabara Bay), 20 min by ferry from Rio's Downtown. It has great promenades (Boa Viagem, Icaraí, Sao Francisco), fortresses (Santa Cruz, Sao Luiz), beautiful belvederes (Parque da Cidade, with a great sunset and breathtaking view over Rio de Janeiro) and a museum of contemporary art with unique architecture:

http://www.macniteroi.com.br/

For my taste, Niteroi's beaches (Camboinhas and Itacoatiara) are much better than Rio's. However, both are far away from downtown (30 min by car with no traffic jams).

I believe Niteroi is a great choice, if your guide knows it well.

I suggest Serra dos Orgaos National Park, near Teresopolis. Great easy hikes through tropical forest, with toucans and monkeys, beautiful trees and orchids. Amazing that such a national park is less than 1 hour from downtown Rio!

If you choose between Niteroi and Petropolis, I would choose Niteroi. I didn't really like Petropolis; the imperial museum wasn't all that interesting compared to other parts of the city and state of Rio.

I regret taking a day trip to Petropolis. Niteroi was much more enjoyable and closer.

Petropolis is mountain. Better to enjoy the beaches. Try a day trip to Itacuruça! Niteroi is not beautiful, has just wonderful view from Rio. Te beaches are not so beautiful as Itacuruça where you can find so many islands and enjoy your day by boat . Enjoy yourntrip!

Thanks all, so helpful!