Less popular now than they were but still available and widely used are the annually released York Courses.
These have a tried and tested formula with an audio CD of discussion starters featuring well known Christian contributors from a variety of traditions and denominations. A transcript of the CD, and a course book for participants allows participants to engage with the discussion they are listening to.
Each York Course follows its own theme and it may be worth looking at a couple before making your mind up which one you might use. We have a few years worth in the RRC!
If you have used one of these in the past do leave a comment so other members have an idea of how successful they may be.