Topic #1: Compliment something. Whether it's their hair, makeup, outfit, or the turkey, drop a nice (but short) compliment about it. Who knows - you may find that y'all something in common you didn't know you had.
Topic #2: Whatever the majority of your family loves. My family (including relatives) are a bunch of geeks so whenever the topic of Star Wars is brought up, you can bet that it's going to be a fun conversation over dressing.
Topic #3: How about what everyone is thankful for? Sadly, whenever I visit my grandmother's at Thanksgiving, we have never done this. I wish we always had. But you can never go wrong with that topic.
Topic #4: Talk about vacations, but don't brag about it too much. Not everyone can take a nice vacation, and hearing about your swanky room and how much money you spent might make other guests feel uncomfortable. The basics will work!
Topic #6: Pets should be one of the safest topics there is. But in the case that someone has recently lost their pet, tread lightly. Otherwise, boast and brag about Fluffy's new favorite toy or how their vet visit went.
Topics to avoid: relationships, jobs, finances, weight, and any topics that may cause an uproar. Oh, and no "one-upping" each other. It's just bad manners and kind of spoils the mood. Trust me on this, please!