My first blog went through a few iterations before becoming what it is today, Colorado-Wilderness. At first I thought it would be about photography, but when I started a full-blown website devoted to photography, I figured this should be something else. I go hiking almost every week in the summer and fall, and like sharing my photos from the hikes, and it's easy to write a few lines about the place I went, so creating a blog about hiking and camping Colorado seemed like a good idea.
Notice how I make full use of the text boxes on top and in the right sidebar to work in keywords about hiking, camping, backpacking, etc. Between all that text and the content of my posts, it's not hard for AdSense to figure out what type of ads to place there.
Other things to note include my use of affiliate advertising and the pages feature. At first I just had single entries devoted to camp food, backpack checklists, etc. but I made them into pages to visitors could find them easily. I think stand-alone pages are easy for search engines to find as well.
This blog is a consistent earner for me, and will continue to be one if I keep posting with any regularity.