<insert joking tone of voice>
Not only do you need a power converter to 24 volt DC but you will also need to start learning Greek very fast as your laptop will automatically detect it is in a different country and all change all the language settings and keyboard layout as soon as you step of the plane. This is why international travel is so demanding and needs loads of preparation.

<end joking tone of voice>

Seriously speaking, answering your question is like answering "how long is a piece of string?"

What kind of laptop do you have? Does it have wireless connectivity or eathernet or dial up modem only?

But before you get in to that I suggest you follow previous advice and check the powerbrick to your laptop, get a plug adaptor so you can stick it in the wall (if it does cover 110 - 240) then my advice is to start trawling google for information on internet cafe's and also check if the hotel has internet.

/E - trying to figure out how long a piece of string is