Depends on how much trust you have in the recipient of the laptop and your technical skill to reformat the hard drive and re-install the operating system (O/S). Overwriting the whole hard drive would not leave the laptop in a usable state until O/S was reinstalled.
Clearing your "My Documents" folders is obvious but there may also be other folders containing emails, address books, etc. Also clear any temporary files, cookies, passwords etc.
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