We flew from Juliaca, Peru to Lima and then had an overnight flight to Rio de Janeiro, Brazil.  From there we got a domestic flight to Foz airport, Brazil at Iguazu Falls.  The Falls are between Brazil and Argentina with Paraguay close by across the Parana River where it meets the Iguazu River.  We stayed on the Argentine side.  Close to our hotel was a little ferry that goes back and forth between Argentina and Paraguay.  We inquired of several people about the ferry and if we could go to Paraguay.  No one had a good reason why we should not go, so we went through Argentina immigration and checked out of the country, got on the ferry.  The fare was only a few dollars.  When the ferry was loaded with 15 or so vehicles and several people, we went on the 15 minute ride to Paraguay.  We went up to the immigration office there only to discover that USA citizens are supposed to have visas for Paraguay.  We had no idea.  We had been told that Brazil was the only South American country that required visas of USA residents.  One cannot buy a visa at this immigration office.  So what to do?!?  The immigration officer was a very nice and kind man.  He was not sure what to do with us, but he did not want to arrest us.  Finally, He said "No problem", and  he stamped our passports with the Paraguay stamps, and told us to go and enjoy the rest of our trip,  and we went back and got on the ferry going back to Argentina.  So that was the end of our Paraguay adventure.  We were delighted to be able to visit both Argentina and Brazil sides of Iguazu Falls.  You will see many photos of  the Falls as well as many critters that we saw in the area of the Falls.  There are spectacular views of hundreds of  Falls everyplace you look.  We also got drenched with spray in certain areas of the Falls. 
It was just wonderful to be able to visit there.  It was very hot and humid  too.  During our whole trip we went in and out of Argentina 5 times which was more than another other country we visited.