Your choices depend on you. Being from an engineering background, you might want to check out Physics, Additional Math or Applied Math. If you aren't comfortable with those subjects, you can look at International Relations, Political Science, Sociology, Gender Studies, any History subject, Criminology, Anthropology, or any Regional language. All these subjects are broad, general and comprehendible for beginners.
However, optional subjects must be chosen carefully based on your own interest and aptitude. Others can only guide you to a certain extent, but only you know what is best for you. So, look at the syllabi of different subjects and choose those that seem easy to you.