Upvote:7
I made exactly this trip in 2013 (except from São Paulo and not Rio). I didn't find I needed a car: fly to Foz do Iguaçu, get bus 120 (or a taxi) to the centre of town.
There is a website devoted to tourism at the Itaipu dam. Plenty of tour operators in town will sell you a tour, but you can also buy tickets online there with a credit card. I took the Circuito Especial which shows the dam inside (including a view of the control room and the turbines) and out, where you can see the run-off channels). You can get there from Foz do Iguaçu by public transport as well: the Conjunto C Norte or Conjunto C Sul lines take half an hour.
It's also only a short bus ride to the Iguaçu Falls: take the bus with Nacional Iguaçu to the end of the line (30 mins or so). This thread gives some more advice about crossing to Argentina if that's what you want to do (I didn't, myself). The short version is that you'll need Argentinian pesos for the entrance fee and can't get them there.
I speak no Spanish or Portuguese and had no problems. You'll be fine.