argentina
The Sheraton hotel right in the park provides a good alternative to the junk food stalls located throughout the park. There's a nice terrace you can rest from which you can see the mist coming out of the falls and also generally see toucans and other birds flying around.
There are some very good restaurants in the town of Puerto Iguassu, serving a very good choice of Argentinian wines.
brazil
On the Brazil side, there's a buffet that stands right next to the throat at the Porto Canoas station at the end of the walking trail. The food is not good but the view of the river makes for a surreal sight as you know the falls are really close by but you can't really see them other than the mist and the noise.It's a nice place to eat. In addition to the buffet at R$ 40 you can grab a burger combo meal at one of the outlets right in front of the buffet restaurant for around R$ 10.