Keep learning new things in free time as you will be having plenty of time to advance your interest.
Now coming to the point. I find it important to inform you about the most suitable subjects you should take that may also help you for CSE Preparation. You can pick from following five:

• Geography
• Political Science
• Economics
• Modern History
• Sociology

You can also take any literature, philosophy or psychology to expand your mental horizon.
The beauty of Humanities lies in its expansion. So keep up the hard work and enjoy your academics
Hope it helps!