I read a tip a few months ago that women should always pack a "pashima" type shawl (approx 80" X 20"). This serves several purposes: it can be rolled into a pillow for your neck on the plane; It serves as an afghan for your cabin and/or wrap yourself in it on your balcony; And, of course, as an evening wrap.
I just found a dark purple soft, thin chenille-type shawl (with fringe!) from Sears just 3 weeks prior to our cruise (Feb. 8). It was $40 and marked down to $14. I chose a color since most of my evening clothes are black. My 12-year-old daughter enjoyed borrowing this shawl all week long, too. I will certainly add this item to my official packing list. It didn't take up any space at all.
I also swear by my 'Bucky' pillow (filled with buckwheat hulls). Not only is it the best in the world for flights (the airline pillows gross me out - they never get cleaned) but it's so great to have if the pillows at the destination/on board aren't up to snuff. I'm picky about my pillows (nothing like having your sleep ruined by a pillow that's too firm/flat) so the bucky is like an insurance policy for a good night's sleep. They are available online (see http://www.bucky.biz/eu/where.aspx or www.ebags.com) and cost more than the inflatable ones, but they are SOOO comfortable. They also unfortunately take up more space, but I think it's worth it.
I also have a pashmina shawl which is fantastic (it's synthetic, so was quite reasonable) for a shawl with any outfit, even formal (be sure to buy black). But it can also double as a pillow or even a little blanket, since it's so big.