I've never tried these, but I remember thinking they'd be perfect in such a situation. Mostly because they require zero fuel (well, hydrogen for a fusion reaction, but you don't have to supply your own), and they can be made out of darn near anything (my favorite so far is the two pieces of glass and an old tire):
Solar ovensCan reach very respectable temps. Enough, IIRC, to bake some things.
Also, dutch ovens look pretty snazzy...
Dutch Ovens In principle, it's just a specially shaped metal pot you fill and then surround / bury in hot coals. Apparently they can bake bread and stuff. But they're less of an improvised oven, because you really oughta already have the pot.
After that, I dunno... Aren't there some breads or bread-like things that can be made by wrapping dough in wet leaves, then covering the leaves with coals? What's the oven for? In an emergency, cookies would be nice, but hardly a necessity. <img src="/images/graemlins/smile.gif" alt="" />