Hi,
I just booked a trip to Iguazu for the beginning of January, i.e high season. I used travel agency called Mawa and there my contact was Yanina Gadeau.
This trip costs 680 USD and it includes flights with LAN, 3 nights in 4* hotel and 4 excursions. One of them Jesuits ruins.
I had quotations from 4 different agencies, this one had best combination of excursions, flights and PRICE!
My colleague recommended this agency so I think they must be at least ok. They have offices around the city but you can also deal with them via mail or phone.