One of the first things you can do is so simple, many renters forget it: install and check smoke detectors. Make sure that you put smoke detectors the building according to your local fire department's suggestions. If the building already has smoke detectors in it, make sure to test them every month. If you have new tenants moving in, replace the batteries and test the detectors before they settle down in the building. Encourage them to test them when they change their clocks for daylight savings time.
Another simple thing that you can do is to supply a couple of fire extinguishers for your rental property. Although these are a little more expensive than other items on this list, they will still only run you an average of $40 to $50 which is a small price to pay for safety. Simply check to make sure the pressure gauge is in the green and there are no breaks or cracks in the hose to be sure that it is functional. Often, landlords will put one in the kitchen and one at the other end of the apartment, perhaps in a closet.
Another thing that you can do before a tenant moves in is purchase and install a carbon monoxide detector. Although carbon monoxide is less common, it still accounts for an average of 430 unintentional deaths annually in this country alone. Since carbon monoxide is colorless and odorless, it is likely to go unnoticed unless there is a detector in the building. They cost around $20 to $25 on average and may also be mandated by your town. There are even some that are smoke detectors at the same time.
These are only a couple of ideas, but they are some of the easiest and quickest to do in order to keep your residents safe. Make sure that everything is functional before they move in, though.