What is the best way to get / spend money in Argentina? Do they take US $$ or do we need get money from the ATM? Any tips to avoid getting ripped off?
They use the Argentinean peso, not US dollars. They have ATMs like everywhere else. Small coins and small change may be a real challenge to get, so try to a void having large bills in the first place (they have a real shortage of small coins in circulation). Not sure how you envision getting ripped off, but I had no unusual problems there in two weeks of DYI travel (this was a few years ago, around 2009). The only thing I would watch out for is making sure you note explicitly how much you give to a taxi driver and making sure they don't try to trick you into thinking you gave them "not enough". Some of the cab drivers at the airport were very aggressive, so just know the approximate fare and try to give them close to that amount so there are no problems getting the right change back, etc.
It may be helpful to read about their default back in the early 2000s and their money issues every since in any economic magazine or piece (The Economist, Wall Street Journal, etc). I don't think you'll be much affected as a tourist but it's good background info and you'll know what to expect. Also, check out Travel Advisor forums to see if folks have experienced any issues more recently.
This is not a rip-off warning, but be really careful with even minimal rental car damage if you choose to rent a car. Unlike in the US, they checked the over with such care and attention that I got fined for damages (dings from traversing on rocky terrain and door issue due to unusually strong wind that almost blew door off while in the northwestern Andes).
You should try to avoid getting stuck with large bills, as even the better shops could have difficulties in giving your change back.
Pay attention when paying taxis. An Argentinian lady told me to make a mental note of the bill serial number (no need to memorize it, just the last digits could suffice). A relatively common trick: you give the taxi driver a bill, the taxi driver exchanges it with a fake one and gives it back to you complaining that it is fake. You take the fake back and you give the driver another good one. But if you remember that the last digits on your bill were XYZ, you can spot if your first bill was exchanged with another one.
It is difficult to give money tips for Argentina as the situation may change very quickly. The last time I was there (August 2012) there were a lot of aggressive money changers in the centre of Buenos Aires looking for foreign tourist and foreign currency. You should better avoid them as they could load you with fake bills.
Other advices from the same lady: pay attention to purses in restaurants. Pay attention to luggage when arriving or departing from Your hotel.
You need to take in account the many kinds of dollar exchange rates that currently exists in Argentina:
If you use your credit card, debit card, or ATM withdraw, you will be charged at the "Dolar Oficial" rate. The real conversion rate is the "Dolar Blue" or "Dolar Paralelo", which is the free market rate, but, is not "officially" allowed to change dollars outside the "Dolar Oficial". When I travel to Argentina there is no problem to me, because I live in a nearby country and I can buy argentinian pesos (AR$) directly from my local currency in my country before I leave, as there is an important market to buy AR$. In that way, I buy AR$ at the "Dolar Blue" rate.
Conclusion: You'll pay about 30-35% more using your credit card or ATM/debit card in Argentina, so its better to get cash at a better rate BEFORE your trip if you can do it so at your country. Just compare your rate with the "blue" and the "oficial".
At downtown Buenos Aires there are many street money changers, called "arbolitos". You can get a near "blue" rate with them, but, as others said, is risky (safety and counterfeit bills) and (I believe) is ilegal, even doe every one do its there.
I always carry all the AR$ I need for my trip because of the above as a I said. Once I got a car ticket fine and asked in my hotel how can I exchamge my emergency dollar cash (yes, USD, not my local currency. USD is easy to exchange almost everywhere and is Argentina favorite saving currency) . The hotel lobby concierge itself offered me to sell his AR$, and he gave me a better rate (even better than the "blue" rate). He even make a "delivery" money exchange to my room (I presume to avoid the hotel cameras). As you can see, USD is very appreciate for the common person in Argentina because of the government restrictrions for them and AR$ volatile value.
Another option and legal way is to buy your pesos at Uruguay or other nearby countries before entering (if you are coming from other South American countries). There are even short day trips to Uruguay from Buenos Aires. Simply sell your USD and buy pesos uruguayos, and then exchange your pesos uruguayos to AR$ in Uruguay and you'll get the better "blue" rate or at least close to it in a legal way, as there are no legal restrictions there. I don't know the rates there but I assume you get "blue" or better than "blue" in a regular exchange office or bank.
I hope it helps you to decide what to do for your trip
Thanks for the tips!
You're welcome, any question just ask.
I just edited my post. I forgot to include that ATM withdrawal (like credit card) is automatically converted at the "oficial" rate, which is not convenient for you (better exchange rate is "blue" for hard cash)
Wow! Things have changed since I was there (it's sounding a lot more like Venezuela)...here are some interesting articles:
"Arbolito" = "tree". In other words these moneychangers hang around with no visible function, like trees. It's about the only amusing thing in Argentina's perpetual currency crisis.