What could you possibly mean?

A carry on bag is for thing that you will need in the airports and the plane. I take things like a netbook, ipod, book, eyeglasses, money, jewelry, passport and all the other accoutrements a woman needs. They are too numerous to type out.

And you're thinking about this a year ahead?

Clothing and necesseties for 2 days in case your luggage gets lost (failure to do this made me miss a trip to Cinque Terre in Italy); toiletries and medications that yu can't get in Europe (most likely you cannot get a non-EU prescription filled); reading materials or electronic tablet for wather forecasts and other uselful apps; a waterproof windbreaker or at least an umnbrella; patience at the airports.

You seriously can't figure out what to pack in your carry-on bag? As apart from your checked bag? Have you never traveled before? Your carry-on bag contains the stuff you need immediately, plus a change of clothes in case your luggage gets lost. It should be pretty obvious.

and snacks for the flight

Electronics(camera, iPad, iTouch, phone) medications, handi wipes, Evian refresher spray, eye shades, ear plugs, jewelry, passport, i.e. valuables. Sweater and/or pashima.

warm socks for the long flight, contact case if you wear contacts, headphones - anything that you can't risk checking.

Also things not allowed to be in checked luggage per TSA such as loose lithium batteries: http://www.tsa.gov/traveler-information/safe-travel-batteries-and-devices

As of January 1, 2008, the Department of Transportation (DOT) through the Pipeline and Hazardous Materials Safety Administration (PHMSA) no longer allows loose lithium batteries in checked baggage.

TSA is applicable if you are coming from the US.

Sizzlesteak, I always include a nightgown, extra undies, and a change of clothes in case of lost/delayed luggage – along with medications, glasses, and necessities mentioned above.

40 ltr is pretty open size wise, make sure that your bag conforms to the Smallest allowed size given by potential carriers, Regional Airlines have maximum permitted sizes for carry on baggage and everything including purses, cameras and duty free shop bits must fit into that one piece of baggage which must also be under a specific weight allowance.

Basically your carry-on must meet dimension and weight requirements, and should contain anything of value you are taking with you on your trip, plus anything you feel you may need during the flight.

As Tony2Phones says you need to check your 40 litre bag actually meets the size requirements of any and all airlines you are likely to use. With increase use of carry-on only airlines are getting stricter as to the size and weight of what you can carry on board. Some will allow a "personal item" as well, some won't. Again you need to check that with the airlines you intend to use.

If I am going on a trip with photography in mind then my photo back pack loaded with photo gear is my carry-on. My netbook, e-reader, passport etc fit in it too, just.

If I am just going on a relaxing, do as little as possible trip then my carry-on consists of my camera with two lenses, netbook (if away more than a week), e-reader, glasses, any jewellery I may be taking with me, passport and money.

I never pack spare clothes in my carry-on. I want it to be as light as possible, and if the whole camera pack goes there's no room for clothes anyway.

Lithium batteries must be in your hand luggage, and must be individually packed so they cannot short.

Personally, my carry on is my bag entirely. So I would include everything I would need for the trip. With a bit of creative packing (ie socks inside of shoes, flats inside of tall boots) you can do it easily for a week. My biggest items are my running shoes and mini iron (since many places don't have an in-room irons - but they do have hairdryers, so you don't need to pack that)