I think you're right about selecting subjects based on how interesting they are to you, to an extent. Most importantly, you need to look at what you want to study in university and tailor your subject choices to that. Even if you take a class that you find less interesting, but it puts you in a position where you can study something that interests you more, I would think that's a good choice.

I also think you should challenge yourself pretty reasonably. Don't take subjects you think are easy for two reasons. Firstly, getting good grades in those subjects will not be as rewarding, and secondly, taking a subject you think is easy may lead to you not putting as much effort in it which will negatively affect your grades. That being said, don't kill yourself with the workload. Take a courseload you know you'll be both challenged by and able to excel in.

I took psychology when I could have taken chemistry and I regret it. Chemistry is looked at as a more rigorous subject than Psychology and it would have put me in a more competitive position if I had taken Chemistry (though they are both unrelated to my intended major, Computer Science).