1) Eat your lunch. Sometimes it can feel like you aren't working if you are also eating, but that is not the case. The more natural the setting is, the more you can model and facilitate language functionally.
2) Don't just talk to your student. One of the best parts about working with this age is that in preschool, speech kids are superstars. I get asked constantly "can I come with you?" "When will it be my turn?" "Where are you going?" - it is not uncommon for tears to fall. Naturally, the curiosity continues at the lunch table. Take this opportunity to start conversations between students, bridge communication gaps and help the peers learn how to communicate with your 'friend'. This is especially important for children who have augmentative communication.
3) Observe what the classroom environment is like for the student. I was taken aback the first time I had lunch with the preschoolers. The conversation, the spills, the yelling for "more bread", the peer regulators ("you can't do that"), the see-food eating... the word chaos comes to mind. It is the stark opposite of my quiet, controlled office, and understanding that can help me plan for my students to successfully generalize their progress into the mayhem.
4) Connect with the teachers - maybe start with acknowledging how amazing it is that anything ever gets done in a preschool classroom. This can be a great time to talk about and model some of the strategies that they can use to support the student, or find out what you can do to support them.
5) Let there be silence. Part of what makes lunch sessions hard is that SLPs do not like silence. Don't work too hard to create communicative opportunities, instead make the best of ones that occur naturally. Even though they don't compare quantitatively to trials in the office, their functionality is invaluable.
6) Join lunch periodically, rather than each week. Like all things, lunch sessions can lose their appeal if done too often. Try to rotate kids and classrooms to maintain the novelty. Maybe even keep some lunches for yourself :)