Oh my gosh - how FUN! Lucky YOU!
Hmm..don't know where you are, but maybe start w/ looking at the restaurants that you may likely go to based on proximity. Access their menus on-line and see what you can plan out.
How about a turkey burger? Maybe less or no mayo and skip one of the buns (or not!). If there's a Lazy Dog Café around you, they do an outstanding turkey burger on a honey wheat bun.
Or an ahi tuna appetizer as lunch w/ a salad and shared dessert?
One thing I love about Sparks: it's about moderation, not deprivation. Maybe just eat like you normally would have for breakfast & dinner, then get what appeals to you at lunch. You can always eat half for lunch and take half home for tomorrow if the serving is big.
Whatever you do, enjoy yourself as this is part of what life is: enjoying a meal out w/ a good friend!
Remember, this is only one meal out of thousands in your lifetime. Use this time to show yourself you can incorporate normal living into your new lifestyle. No starving yourself pre or post lunch. Just go out and enjoy.