I never understood the adsense game... If you have a web site that draws relevant valuable traffic why would you dispose of it by driving them away from your site? It's silly... you should monetize your own web site to generate profit. Even though this isn't adsense, I have a web site that I use to advertise ads on and I would make a few hundred here and there. Then I changed a few things on the site and started selling a "virtual service" and ended up making much more. Adsense is only useful if you have a high traffic web site where you KNOW your redirected traffic will return to your site!
I have found an article that you might find interesting ... Check out: http://www.listsforwebmasters.com/list/list-of-website-ideas
I'd go for something you're interested in. It will be much easier to keep the website going if you know what you're talking about
I think going with with google trends its a good idea .I would search for trending topics and i would write blogs articles about them