Ignore the first poster. One week => stick to Rio.

Hotel Arpoador is nice for a hotel, Ipanema Beach House for a hostel. Both great locations.

Things to see/do in Rio are endless:
- Beaches;
- Sugarloaf;
- Christ the Redeemer;
- Lapa;
- Santa Teresa;
- Hang gliding;
- A football match in the Maracana.

A recommendation for "a good accommodation in Rio De Janeiro that is affordable" might be easier if you told us (exactly, in reais preferably) what YOU consider "affordable". Rio is relatively expensive (probably over U$200/R$400/night for a "good" hotel in the beach neighborhoods mentioned below) these days, so "affordable" could be a bunk bed in a multi-person hostel dorm room. I see many posters on Brazil forums underestimating by a long shot what accomodations in Rio might cost. Consider going in the off-season, better for sightseeing anyway, if cost is a major factor.

btw IMO the Arpoador is a dump, not good value for money, although in a great location.
The tourist neighborhoods with the most infrastructure to choose from are Copacabana, Arpoador, Ipanema and Leblon.

The site www.ipanema.com has a lot of info on neighborhoods, sights, beaches, clubs, etc. If you put together a tentative itinerary of things that interest you, we may be able to help refine it and add logical transportation routes from you accomodation.

With only one week (many could happily spend the whole week in Rio itself), you could think about a few days (by bus) in Paraty/Ilha Grande or Buzios, or two nights (by air) in Iguacu to see the spectacular falls.

Happy travels.

I haven't stayed at the Santa Teresa Hostel, but it looked quite nice from the outside and it has the most spectacular view over the city.

From there you can easily walk down to the city and if you want to go the beaches area (Zona Sul) you can take the metro from there.

http://www.riohostel.com/home.php?lang=eng

As nice as that rather expensive hotel is, and as good as a meal at Aprazivel, for example, can be, Santa Teresa is just not very convenient. And I still don't consider walking around in Santa Teresa to be completely safe. While you can walk "down" during the day, since the bondinho (streetcar) is not yet working again, you would be taking taxis (some of which even refuse to go up the hill) or the buses to get back and forth. There are plenty of places to stay along the beaches, which are also close to the sights, the metro, restaurants and services. My preference (maybe not yours) is not to ride public transport in beach gear and possibly still a bit sandy. OP, please post a budget range for more specific recommendations for accomodations.

It's the Hostel in Santa Teresa that I suggested, not the very expensive Hotel also in Santa Teresa. I'm not sure of the price but it looked like a backpacker kind of place. The pictures of the rooms looked quite nice of the web.