I usually work with aluminum too. But I've also been known to build frames out of copper water pipe and fittings. The stuff is so much fun and easy to assemble - and you end up with a pretty cool "industrial" look. The copper can be brushed and clear-coated for a nice bright shine. Galvanized water pipe works well too because it threads together. But it's a little harder to cut and you have to make new threads if you do cut it. PVC and ABS pipe also are fun to work with. Lighter but not near as strong or rigid as copper and steel.
I think if you did it right, an open, structural frame made of both aluminum and copper would look sweet.