You could use the Sharp distance sensor in the lid pointing down at the contents:-
http://www.google.co.uk/search?q=sharp+ir+distance+sensor&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#q=sharp+ir+distance+sensor&hl=en&client=firefox-a&hs=SXH&rls=org.mozilla:en-US:official&prmd=imvns&source=univ&tbm=shop&tbo=u&sa=X&ei=Dx1nT4TgMNPB8QO39Py3CQ&ved=0CH8QrQQ&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=55d0202283a838d9&biw=1264&bih=726
if it is possible to integrate those in Arduino boards
No you add them on, unless you are building the equivalent of an arduino board with discrete components.