I suggest browsing the tags list and looking for less popular tags that you have a good breadth of knowledge in. When browsing by tag, use the Related Tags sidebar on the right to narrow. Clicking multiple times here will combine the tags so you get increasingly more narrow in focus.
The unanswered question page is also useful -- and, similarly, try narrowing it down via the Unanswered Tags sidebar on the right. It works the same as the related tags sidebar, above, so click multiple times to get more narrow.