The past perfect can be tricky to explain and even the most advanced students can be inclined to stick to the past simple instead. One example you can give that will help to illustrate the difference is:
when I got to the party everyone left
when I got to the party everyone had left
Write these two sentences on the board and see if students are clear as to the difference in meaning.
A good way to get students to produce imaginative past perfect sentences is to have them explain why they were caught in a certain predicament.
Print out or make up a set of past perfect predicament cards. Give each pair of students a shuffled set which should be placed face down on the desk. The first student takes the top card and reads the predicament to their partner.
For example, I saw you at a party last week, you had a big smile on your face, the second student must now explain how they came to be in that situation by using the past perfect, e.g. Yes, I had just got a promotion or I was smiling because I had drunk a whole bottle of champagne.
Make sure to model this well, students might think they need to explain what they were doing and not what they had just done or been doing.
Finally catch up with some of the most interesting answers from the group and sort out any grammatical problems.