1. Be transparent about who is paying for what. Be mindful that some in the family might not have as much disposable income as others and plan accordingly.
2. Make sure everyone is on the same page about where you are going and what you will be doing when you get there. Many times, families have different vacation styles and rhythms. (Think getting out early as opposed to sleeping till noon...)
3. Make sure grandparents have some down time away from the kids and that there are enough bathrooms!
4. Have a meal plan if you aren't staying some place where meals are included. Who is shopping and cooking? Who is cleaning up? Who is watching the kids so that the adults can have an evening out?  What kinds of restaurants does the group agree on? Most important, how are you splitting the costs? 
5. There is no need to be in lock step the entire time. Even if it's a long weekend trip, plan so that each family has some "alone" time, if wanted .
6. Grandparents: Don't spring for any expensive tickets until everyone has expressed enthusiasm.  You don't want to be stuck because the kids' interests have shifted.
7. If the kids are different ages and have different interests, suggest splitting up for a day's activity.
8. Agree on a uniform souvenir strategy for the trip so that no child comes home with more than the others.       
9. Stay away from topics that are guaranteed to cause friction.
10. Never criticize anyone's parenting style and never discipline anyone else's child. 

Now you're ready for some togetherness-hopefully!