It is possible to do a 5 day, 85 km hike (rated Medium to Difficult) from the top of the canyon down to Ai-ais. You have to be bloody fit to do this hike though. Myself and a few friends are planning on doing it next year April, and it has potential to be the most amazing hike a person could do.
Everyone seems to rush through on their way to Fish River Canyon, maybe spending one night, but it really is a great place to stay for a bit longer. There are loads of activities to do along the whole stretch, but canoeing is great just over the border from South Africa and you can stop off and swim in the river too.
Fish River Canyon if you don't hike you might like to see it from the air full lenght.
Here's an aerial view of the Canyon very impressive a fellow traveller compared it to Bryce in the US.
Company that does the flights, Nature Investments ltd.