I just got back from Iguazu yesterday. As the other posters pointed out, you really shouldn't need anything special except - depending on your complexion - lots of sun block. Also, bring some insect repellent and something to treat bicho bites.
I really wouldn't spend more than 3 days in Iguazu. You only need 1/2 day on the Brazilian side and a full day on the Argentine side. Beyond that, there really isn't a whole lot to do. I took the helicopter ride on the Brazilian side and would suggest it as a worthwhile way to see the falls. It's only 10-15 minutes but the photos are amazing!
It might also be worthwhile to stay on the Brazilian side in Fto Iguazu rather than in Puerto Iguazu. Puerto is very, very small and there really is very little to do or see there. I gather that the Brazilian town has three times the population and something of a nightlife - well at weekends anyway.
If you are bussing it from Iguazu to Buenos Aires, be prepared for police stops every hour or so on the way back - complete with sniffer dogs!