Any suggestion for a science investigatory project?

If you can choose to do a project on anything, that's pretty broad... If it were me, I'd have a think about something that really interests me, and go from there. If you choose something that you have a genuine interest in, I bet you'll find it a lot more fun to work on, and the end product will be better.

I would want to choose something that was impressive to look at, so that people take notice of it. The cool thing about this, is it doesn't even need to be overly complicated. You can get some impressive chemical reactions from basic household items like bicarbonate of soda:

or Coca Cola:

Try a few things out at home on your friends and family, see what works and what doesn't, and how you can apply it within the context of a science investigation.

The more thought and planning you put in beforehand, the better the outcome will be, I guarantee it!