What are after school activities that I should do if I plan to study computer engineering in college

I would suggest trying your hand at writing some code. www.codecademy.com is a good site to try writing some beginner programs and see if it's something you enjoy. Also if you have after-school or summer free time, check with local technology companies if they would be willing to have you intern. Actually being around working engineers is the best way to find out if it's the best career path for you.

If you are still at the stage of deciding whether you want to take up computer engineering, first thing you should do is talk to as many computer engineers you know. Their knowledge/perspective will help you get a clearer picture of what all computer engineering entails. Second, read about the different aspects of computer engineering and see if any part of it interests you. Once you find an area of interest, pick up some classes for it or start reading up more about it. The more information you have the better for your future decisions.

What sort of computer engineering do you mean? "Computer engineering" might mean you're interested in building computers, or designing chips, or it might mean you want to write software or administer networks or servers or design web sites, and aren't really interested in the physics/electronics/hardware part.

The number one activity I would suggest would be to go and shadow someone who is a computer engineer. There are a variety of ways computer engineering can be used after college (your major is a big part of what you decide to do afterwards) and nothing anyone can say will have nearly as much of an impact as seeing firsthand what a computer engineer does.

After that, if you're still interested in the field, I would suggest joining a few clubs or even outside groups dedicated to the various aspects of engineering. There are software and hardware paths, and many sub disciplines within each. Getting as much exposure to these as possible will help in guiding you in your decision on where to go to school, what to study, and what to do afterwards.