Please please please help me out to find a nice topic for investigatory project in biology for class XI (cbse)... please...

No, you don't want a "nice topic." You want something that appeals to you and totally interests you because you will be spending hours on this, setting up the project and doing research, and running tests. Also, it's time you learn how to Google with good keywords so you can do this initial research yourself to find an idea.

Let us know what project idea you decide to do.

Ummm... I want a topic for PROJECT and not for giving to my loved ones. Well thanks for your suggestion.