Have you asked the prof what you can bring? If you haven't been asked to contribute to the evening's menu, then you've been given some excellent suggestions - bottle of wine, coffee, coffee table book, fruit, flowers. If your prof asked you to bring something specific (e.g. appetizer, salad, dessert) and you're at a loss, let me know and I'd be happy to share some of my favorite easy-but-snazzy dinner party recipes.
When I was in grad school, one of my profs had a dinner party for all of us. I didn't have any idea what to take, so I took a wine bottle holder (similar to
this). I also stashed a thank you card in my purse, thinking I might have an opportunity to write a quick note. I didn't have such an opportunity, but I mailed my thank you the next morning. (My classmates called me a brown-noser for the thank you note, but it seemed appropriate to me - his wife laid out an amazing spread, and she totally deserved some recognition for the work she put in for people she hadn't ever met. I couldn't
not thank her.)