Best advice blog about something your interested in, dont just blog for the sake of it you'll find adding new posts a chore and my guess is you'll probably end up abandoning it!
Start blog about subject in which you are expert (interest). As you said you are sitting idle then there are so many ideas comes in your mind rightnow which confuse you so why don't you start blog on that (ideas comes while sitting idle) ?? j/k DON.
Why are you trying to find something to blog about ? Do you already have a blog and just have writers block, or are you thinking of starting one and don't have a subject ?
you can blog about anything you hve knowledge about...any field..your interests..your thoughts..but u want to blog for yourself or for SEO purpose?
blog about "i dont know what to blog" Simply explain in a post this situation like "hey I got no idea"
This can go a long way and I wouldn't take it as joke I've seen several blogs actually "start-up" by writing a "general" or "i dont know" post and it helps things get rolling Or you can check out 7 ways to get post ideas
If you have many ideas to blog, then start your own personal blog and write anything that come across your mind (as long as it's not a bad/illegal one). Topics can be segregated by categories.