Cardboard. It's the stuff they make boxes out of, though if you need an impromptu TARDIS, it's great for that too. Or, if you're feeling creative, you could make a replica of Iron Man's suit. Sure, it wouldn't stand up to explosions, machinegun fire or a somewhat determined cat, but for those few moments before it started to rain, you'd feel invincible. And then soggy.
The suit is the work of one Xhongkai Xiang, who happens to be quite the whiz when it comes to building awesomeness out of paper's more sturdy cousin. According to Xhongkai, it took about a year's worth of work to complete his corrugated Iron Man -- the ribbed appearance is intentional, as he wanted to keep the "cardboard colour and texture".
The Iron Man suit is just the tip of a rather large (cardboard) iceberg. You can check out more of his work over at the Stan Winston School of Character Arts.