I have a small cross the body document holder. The type think you might see a lot of people wearing these around their neck while going through airport security with their passport and tickets. It's nice and flat and about 4"x8" or so. I bought it in the luggage section of walmart. I haven't used it in the past on vacation but wished I packed it on my last cruise to use as a 'purse'. It has a window section that I think would be good for the ID/Room card and a zipper down the long side that would fit cash or maps, coupons, sunglasses etc. You could probably squish in a small sunscreen even. I'm trying to think a someway to bling it up before my next trip to make it a bit more dressy.
I also have a small super light weight backpack (cotton) that I use when I need to take a sweathsirt , Side pocket for waterbottle. I usually take it off the ship with me almost empty in preparation for hands free shopping. It's small enough to use it as my 'purse' for airtravel and still have a carry-on.
Dont forget to pack the sunscreen!