Seeing the falls themselves is definitely the highlight, regardless of what else you do. Truly one of the wonders of nature.
Other highlights were the great atmosphere at the place we stayed (Grubbie's Grotto), the gorge swing, and the river cruise.
You could easily spend well over US,000 on activities if you want to. So before you arrive, make sure you've thought about (and budgeted) what you would like to do - from bungy, to gorge swing, paragliding, rafting, helicopter, light aeroplane, and the list goes on.
Best Way To Get Around:
In town, on foot. Otherwise, you get picked up for any organised activity.