Radish, cabbage, and broccoli, ... Oh kohlrabi too and many mustards will sprout -- i.e. emerge from soil with seed leaves in 2-3 days if kept at upper 70's with plastic cover to hold in moisture and maintain temp. Upstairs and near the ceiling might do it if downstairs is cooler.

Gardeners have used various soil/bottom heating techs in place of the somewhat expensive soil heating cables and pads sold for the purpose -- including placing the seed trays on top and near the back of the refrigerator, top of water heater, heating pad for aches and pains but you need the older kind that doesn't' turn off, waterbed heater, pet bed heater... Etc. I've used outdoor holiday lights --non-flammable surfaces and GFI outlet are a must -- but the new LED lights don't emit heat. Setting the seed trays in a storage tub lid filled with water with an aquarium heater would do the job (or on top of a tropical fish tank). A former Organic Gardening mag editor described using an old ugly living room lamp with a bare incandescent light bulb under a metal ironing board -- soon to be a thing of the past.... I recently saw a heating cable for water pipes that is plugged into thermostatically controlled outlet and wondered if that would work for soil heating and what the cost comparison would be.....
