I love the antique letter here. The handwriting, the stamp, the ink smudge...
Other items they suggested photographing: a pretty flower from your sister's wedding, your grandmothers china, a vintage button package (reminds me of this post), an old ruler, a cool ticket stub that brings you happy memories...
Here are a couple of sources for large-format printing:
U Printing (I've heard these guys are especially good)
Large Format Posters
Mega Print