From least involved to most involved method.

1. Body heat. The lowest of low tech. In my home, this would mean gathering humans and pets into the smallest or second smallest room and hunkering down. With a goodly supply of sleeping bags, traditional bedding, and perhaps some jury-rigged extra insulation over the window(s), this would be a surprisingly effective method in a mild scenario. For instance, in a power outage of 72 hours or less, with cold weather but not subzero temperatures.
2. Candles. This adds a layer of complexity, but is still very manageable. Even one or two emergency candles will increase the temperature of a room significantly. One advantage/disadvantage here is that you also have a light source. Dim, but helpful.
3. Off-grid heating. As promised, the most involved method. One popular option here is a camp stove that runs on butane or propane, or a grill. The obvious problem with both of those is that they are really meant for outdoor use, and getting creative with them can lead to carbon monoxide poisoning. Your battery-powered smoke and carbon monoxide alarm provides some safeguard, but these are probably still best saved for cooking, out of doors. If you have a fireplace, keep in mind the reality that the ratio is approximately 20/80 for heat provided versus heat lost up the chimney. But! If you have a wood burning fireplace, it is pretty straightforward to install a wood burner. This is sort of a compromise between a traditional woodstove and a simple fireplace that is just for looks. The wood burner still lets you have the aesthetic appeal of a wood fire on a chilly night, but reverses that ratio to more like 80/20 for heat provided versus heat lost.