From least involved to most involved method.
- 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.
- 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.
- 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.