https://turismoitaipu.com.br/en/
Get the "special tour" which takes you inside the dam. An absolutely incredible spot and incredible achievement. They will take you into a room with a turbine shaft that's mechanically transmitting 700 MW of power.
Expect potential weirdness though. My wife was wearing a (not particularly short) skirt, and the lady at the office selling tour tickets made her step back and spin around, then said she couldn't go in like that. There is a gift shop that sells dam branded pants, so she bought a pair and we were cleared to go in. After all that, one dude in the control room was watching soft-core porn on one of the control room computers.
The turbine shaft room is especially crazy, since they let you (at least back then) walk right up to a few inches away from the shaft mechanically transmitting 700 MW of power! You could reach out and touch it, but we didn't and I wouldn't recommend it.
You're also invited, but not required, to look over the edge of the dam from the top, which is also very far down.