Ask each of the guests to bring photos of themselves with the retiree from years ago. For instance, ask guests bring a photo of themselves when they were together in college, or were co-workers, etc. Makes for great memories and lots of talking over of old times. If you're having the party in your home or at a restaurant, buffet style is the easiest way to serve food. If you are catering the meal, buffets are often less expensive than a formal sit-down affair.
Snacks are important to provide for guests to munch on until dinner is served.
Even if you've never thrown a surprise or adult party for someone before, ask around for suggestions and ideas. Think back to parties you enjoyed and remember what foods were served.