"Show and Tell", one of the meanings of showing is to illustrate something unfamiliar.
So you should introduce something peculiar to your classmates, something that would fascinate them. Here are some examples that i thought of.
You can do some research on Dolphins海豚, or owls貓頭鷹, because it is something people are familiar but don't usual know much about. YOu can entertain them with some facts about Dolphins.
Here is a piece of article from "Yahoo. Knowledge" that may provide you with some help:
About Dolphins:
http://hk.knowledge.yahoo.com/question/?qid=7006081402732
About Owls:
http://hk.knowledge.yahoo.com/question/?qid=7006101802560
http://hk.knowledge.yahoo.com/question/?qid=7006032000433
You can also talk about another country, somewhere interesting, like Egypt. very fascinating and mysterious country with a long history:
http://hk.knowledge.yahoo.com/question/?qid=7007022304064
Or even corsets in the western counties (this is a topic i used once)
http://en.wikipedia.org/wiki/Corset
Hope it helps. Good Luck.
參考: Own idea, but provided reference websites from wikipedia and yahoo knowledge