If the phone still works, search through the phone book and find a number of importance,such as the owner's mother,home number,or work number.
Dial the number and tell them who you are,that you've found the phone,and would like to return it.
If the phone does not work,then call them from a payphone,and not your home phone.
Ask the person to meet you somewhere to return the phone. (PS) Do not meet the person alone!. You still don't know who they are! (Good Luck)