Both suggestions above are good. One house we did had an outdoor deck area which led to a pool and we put timber decking on the deck with a perpendicular timber-decked walkway leading to the pool. We then put lights in the walkway which created a 'catwalk', continuing along the deck. It looked great at night lit up with the pool lights on too! Love the stepping stones and gravel idea too.

I would inlay it into a frame..ie with a base to hold the marble..then that way you can screw/weld the legs to the base of the frame..

Something like this would look amazing! I'd like to do something similar for my island bench with draws on the kitchen side.

Sandwich panel should be cheaper as you potentially won't require any rafters only a beam at either end for support. You can use quite thin panels as well. Some companies do a 50mm panel that will span 7.00 metres+ depending on wind class. Get the whole job priced before deciding. I've done quite a few and the sandwich panels are usually cheaper as well as providing insulation, less maintenance, etc.