The easiest subjects students can write about in a blog is something they are passionate about; a subject they have a significant amount of knowledge in and a subject that will keep them updating their blog on a regular basis because they want to, not because it's a necessary chore.

A genuine passion in a subject will shine through in a blog and generate more traffic to the posts because readers will be interested in what's being said by the blogger. More eyes on a blog means there is therefore more opportunity for generating extra income.