This is one I'm unlikely to implement, because I have more sanity than money, but here it is:
The largest 52 metropolitan areas in the world, one a week for a year!
The reasoning goes like this. I like cities, and I like travel. I've been to a lot of these places, and I'd like to visit most of the ones I haven't been to yet. (Clearly, there are exceptions: for example, I won't be going to Baghdad as a tourist until the security situation improves.)
Wikipedia and world-gazetteer.com both have lists of metro areas, which of course conflict. In fact, Wikipedia has two: a list of urban areas by population and one of urban agglomerations. (I prefer the former, because Boston/Providence is on that list at #37, while on the latter Boston is way down at #56. On world-gazetteer.com it just misses the top 50.)
Needless to say, this trip would be very expensive and quite a test of stamina. Even if I didn't visit them in order of population, but chose some (relatively) sane geographic order instead.
But boy, imagine the food blogging!