Rubbish is perhaps the greatest friend of the historian .
Ask students to think about what they put in their rubbish bins in a typical day.
Then ask them to consider what an archaeologist might think of that rubbish in a few centuries.
Encourage them to think laterally - if future communication is all based on voice, or even telepathy, what would our future archaeologists think a pen was used for?
Some good ideas can be found at the Council for British Archaeology Young Archaeologist Club: www.britarch.ac.ukyacindex.html