What word could you use to approach your friend in Chinese with? Either close friends, family friends or fun/casual friends. E.g. "hey ... (close/family/fun friend), what are you up to?" I am looking for a word to replace the "close/family/fun friend" with.
Words you could use to approach your friends in Chinese with are: 

老爸 (lao ba) for a dad or 老妈 (lao ma) for a mom. 老兄 (lao xiong) and 老哥 (lao ge) for older male persons and 老姐 (lao jie) for an older female person. 老弟 (lao di) for a younger male person and 老妹 (lao mei) for a younger female person.
In all of the mentioned examples, 老 (lao) doesn't mean old. Its closest English equivalent might be "dear". As in "dear ((younger)(fe)male) friend", "dear mother" etc. 

Another way for approaching a good friend with whom one shares a lot of interests is 好小子 (hao xiao zi), which is an expression used irrespective of age.