something interesting比较常用,因为something是不定代词,用来修饰它的形容词要后置,因此something interesting表示"有趣的东西或事".
interesting something用得很少,一般只出现于特殊情景下,此时something只表示"类似...的","有些...的","...那样的"之类的意思.比如:
This plan sounds interesting something.这个计划听起来比较有趣.
"正对某事感兴趣"个人觉得应该说成 Sb. is being interested in something. 因为interest是及物动词,但意思是"使某人感兴趣",就是说它后面带的宾语应是somebody(人)而不是something(事).总而言之interesting something用得比较少.