I've been to both Rio and Buenos Aires - here are my few tips:
1. Do not, under any circumstances, hail a cab. There is a good chance you will be taken somewhere as a set-up and robbed. Hire all of your drivers through your hotel, and never waver from this.
2. Do not carry large amounts of cash anywhere in Rio. When you go on to the beach bazaar, only bring limited funds. Everything there is negotiable anyway. You can do a currency exchange at your hotel.
3. Take a camera, sunscreen, and a banana hammock to fit in.
4. Try to make it up to Cristo de Redento at the top of the mountain overlooking Rio - it's incredible and not to be missed.
5. Definitely go to Meli Melo (Boys and Girls) - a very hot club where you pay roughly $25 to get in and get a card worth a lot of drinks - the bartenders stamp it each time you order, so you don't need to be always paying cash. Drink a lot of Caipirinhas.
6. I stayed at the Copacabana Palace right on the beach - it was awesome. We walked across the avenue and we were in the ocean.
7. Try to know cursory Portuguese - it is close to Spanish, but if you can learn a little, you won't be taken as much advantage of.
8. Buenos Aires feels like NYC - it is packed and energized all the time. I stayed at the Hyatt downtown, which was very nice. The currency back then was pegged to the dollar, and they have had a lot of economic turmoil in the past few years. I felt very safe there, but it might be different now.
9. Use a lot of common sense, which will be in very short supply during Carnaval. A lot of the street thieves make their year during this time, so just be careful.
10. Be prepared to be absolutely floored by the quantity of stunning women, all in Brazilian cut bikinis (if anything at all). It will knock your socks off!
BTW, I read that Argentina is putting American tourists through some additional security steps now to make it even with what we do - be patient.
Enjoy!
Todd
