One of the first things you can do is widen doorways. Doorways, especially in older buildings, are often not wheelchair or walker friendly. This can present a major problem for basic mobility around the house. Also, if there is a lip at the doorway, try removing them. Trying to get a wheelchair up and over them may not be difficult when someone is pushing your chair, but if a person must do it themselves it can prove to be a difficult and sometimes dangerous task.
Another thing you can do is modify or get rid of any stairs you have. If you are renting out a two floor home, look into the installation of a chair lift. If there is a specific resident that is asking for this, you may even be able to split the cost with them depending on the situation. If your floor plan has two or three stairs to get from one room to another, contact a contractor and see if there is anything that can be done to make them more handicap accessible. It is a lot of work, but it should help you in the long run.
With the size of senior citizens growing, it's no surprise that more and more are looking for places to settle. You will likely have a leg up on other property owners if you make sure that your location will accommodate the needs of those who are beginning to grow older. Ask around and see what else may be useful to you when trying to make your apartment more accessible.