This is purely on your suggestion of "what you know to be the shortest thing". This may, or may not, seem a bit outlandish but there is a reason it is so lucrative. So my answer would be porn.
I read a figure that said something like 70%+ of ALL Internet searches are related to porn, sex, or nudity. Not that I have really given it much thought, but I am positive there are dozens of ways you could get to $1million in porn. I have a couple off the top of my head:
1. Tube site: I'm sure you could get video crawlers that take videos from other porn sites. You could go on eBay and buy views for all of those videos to inflate the perception of the traffic you are producing to other users. Then you could use an ad network for popups, banner ads, or whatever.
2. Cam site: These are supposed to be super popular, as pirating (although still an issue I'm sure) is much harder. There are numerous examples of popular cam sites, but because it is "porn" all of them are shady. There have been numerous complaints filed against the owners of myfreecams for example. I guess for this after you develop the site, you could use Craigslist to find models. Drive traffic to the site (again, it's porn, so marketing naked girls would seem to be easier to market than some eBook), leverage social media, or whatever else. You could also promote traffic for some other site, but affiliate marketing is overrated in my opinion. No offense to anyone doing it, but it just seems like an awful amount of work for a measly scrap of the revenue you could be seeing if you owned the site/content or whatever.
3. Produce your own content: Now this does need some overhead. Cameras, a somewhat trained crew, models, lawyers, location etc. This one would be what I would stay away from. Too much headache for me personally.
I am not saying turn into a "smut peddler" or some porn kingpin, but if your sole goal is to get to $1million as quickly as possible, porn seems like a great solution. Porn is recession proof, there will always be a demand, and you can't argue with the numbers. Best of luck.