Good stuff. StumbleUpon is only the beginning if you're actually going to organize some serious promotion, though.
Start with social bookmarking: put together instructions similar to this for Digg, Redditt, Delicious...heck, just give everyone AddThis (
http://www.addthis.com/bookmark.php) and tell them to sign up for and bookmark Elements on every social-bookmarking site on there.
Then, organize a minipage fest. Build a Squidoo page about how awesome Elements is (Here is a sample (
http://www.squidoo.com/tactics_arena_online) that I built about the last game I was obsessive over), and have everyone build smaller fanpages on Squidoo, Weebly, Hubpages, and Terapad, all linking to the Squidoo page. It sounds weird, but the traffic to the game will actually be better if you have the smaller pages link to the main Squidoo page than if you link straight to the game's site.
Finally, get some people to go to the big forums that have gamer orientations or subsections, and post. The IGN boards, OffTopic.com, SomethingAwful, ZenHex, GameFAQs, DigitalSpy, NeoGAF, GameSpot, File Front, and so on. Register, make a sig about Elements, and just plain post in hot topics to maximize the number of people that see your sig.
Those are about the most effective free marketing techniques out there.