Going back to what d9 said somewhere earlier, 1-5 days with no light would really not be a problem at all, I promise, your plants will live! Early on in my planted tank rookie days, I did a 5 day total blackout to get rid of green water, and I did not lose a single plant, Scouts Honor.
Insofar as heating goes, I think the best suggestions made in this thread are the propane/kerosene heaters, followed closely by the sheets of insulation. Temp will be much more of an issue for the fish, plants will survive a couple days with water temps even down to around 55 or so.
Some of the other things, the candles or sterno, well they scare me. Maybe I'm extra paranoid about this, but having survived a small house fire (mostly smoke, but still pretty scary), I try my best to avoid open flames in my house. Just keep safety first, whatever you decide to do!