Don't forget you passport!!
Remember that the argentinian and brasilian borders are really close, maybe you want to explore Puerto Iguazu and/or Foz de Iguazu.
If you go to Brasil by bus nobody is going to ask you for the passport but you could be in trouble if they ask it to you in Brasiland you don't have it with you.
To go to Argentina the passport is absolutely necesary.
Fun Alternatives: Fotokopies are not a good alternative.
As soon as you arrive in this region (you'll probably fly into Foz do Iguacu in Brazil), you'll notice billboards advertizing the Mona Lisa Casino in Ciudad del Este. It was funny to finally see the real deal when I arrived. It's a dirty building filled with shops and is supposed to be one of the largest shopping centers in South America. That fact, I highly doubt. It's definitely not one of the nicer malls. And, I never did see a casino!