A couple of other companies have also offered backpacks that convert to panniers. (Or panniers that have shoulder straps and convert to backpacks.)
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Another possibility (I haven't tried it, but I have seen it done): two milk crates, mounted on both sides of the rack (like panniers), using some good thick hose clamps to secure them to the rack.
You get the weight down low, and double the capacity, and leave the top of the rack free. You just need to watch for heel clearance (as you are pedaling), and mount them accordingly.
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One of the best, most useful things I have found for a bike is a good, strong rack.
The expedition racks made by Jandd are probably the best ones in their price range. They have great top and side platforms, and are made to be extra strong and reliable.
Blackburn expedition racks are also good for their price range.
(I've used both, and prefer the Jandd. It's a great rack.)