years ago and things might have changed, but from the Argentine side you could go out on a walkway for a good view of the Devil's Throat, the biggest and noisiest bit. You can also visit some of the smaller and prettier side falls, and even walk across the top of them (water flow there isn't fast or deep).
From Brazil, you can go down a path by the hotel and look up at another side fall, one with a heavy flow. It's worth doing this if you have the time, but don't fret if you haven't, you'll see plenty on the Argentine side.
Helicopter flights may be available on one side or the other, and they provide excellent views (assuming they keep their windows clean).