You could do a lace handkerchief. Some very fine silk, rayon, tencel, or a combination of those would work well, as would a very soft wool.
There are many pattern that would adapt to this (anything you could knit in a square or whatever shape you want). I think something with a celtic knot in the middle or around the edges might be lovely.
At the bottom of the page on [url=http://www.earthheartdesigns.com/Earth%20Heart%20pages/Patterns/All%20Patterns/D&F_shawl.htm]this pattern they show a photo and mention how you can take the shawl pattern and use a small part of it, edge it and use it as a bride's handkerchief.
Can't wait to see what you come up with!
Oh... any stitch pattern book, Barbara Walker, the Vogue Stitchionary, whatever... could help you find ideas.