George from Edmonton, Alberta has a way of slowing down the evaporation of water from any drain that is rarely used, and hence put off the appearance of sewer odours in that room. He simply pours a thin layer of oil over the top of the water in the drain. Any oil will work, but mineral oil will not go rancid nor hurt the sewage lines.
For the basement floor drain that could sit there for years with no water, there is a company in Winnipeg that produces a whole line of dry drain traps called "Dranjer". Check it out by clicking on the link Dranjer.