The public lecture should be given a flashy, popular title. It should be identified in advertisements as "for the General Public," or similar.
Visual aides (eg. photos, professional quality slides or viewgraphs, etc.) and analogies, stories, etc., should be used effectively by the DTL.
Have the lecturer send a reading list of accessible papers for students to look at ahead of time.
Have a session for students called "What is it like to be a research scientist?" or "Graduate School and Beyond, the Future for Physics," etc.
A past lecturer (Ron Shen) presented a special talk for undergrads titled "The Simple Ideas behind the Nobel Prizes in Laser Science."
Free-form discussions with groups of undergraduate and graduate students might be difficult to pull off. It is better to have structured topics. Example — a guest lecture in a class related to Laser Science.
Lunch with students might be a good idea to provide an informal meeting without pressure.
Dr. James Kafka is a noted senior scientist at Spectra-Physics. He has led development on many cutting edge laser technologies such as the Millennia X lasers shown here.