You can create a blog on high paying Adsense niches such as health, Insurance, Finance, Personal Finance, web hosting, domain names etc.
Those are all high paying niches in Adsense and will earn you top dollar for each Adsense click.
I would make a website that discussed more than one topic so you won't run out of topics to blog about and you'll have more content on the website.
You can create more than one website but I would find it easier to manage one big blog that had articles on more than one topic.