Depending on your requirements you might not need to use code as an OOTB smartobject Activity Instance (Activity Instance Destination might also be of interest) provides those details and allows you to filter by Process Instnace ID.

Try running the SmartObject Tester tool under neath the program filesK2 blackpearlBin folder where you have K2 Studio installed (assuming 4.5) and then in the SmartObject explorer expand Workflow Reports/Workflow General and choose to execute the Activity Instance smartObject. Using the tool is a great way to see if it'll meet your needs without doing a bunch of coding first.