A successful family trip, I think, is one that kids and parents feel equally passionate about; one that allows both their own spaces. Cruises are great, because activities are nonstop, and parents and kids can be as together or as alone as they want. Ranch vacations are ideal because everyone stays active and goes to bed tired, and there are plenty of activities just for the kids, so mom and pop can have some time alone. I recommend just about every type of active, outdoor trip, such as river rafting, biking, horseback riding, llama trekking, and camping--trips that bring families together and let the kids direct their energies at life, not at each other. Have you ever cooked breakfast with your kids over a fire--I can't think of anything more intimate.
One of my own favorite family adventures is backroad biking because you can cycle in and out of each other's lives all day, and see more than you can on foot. It's also something your kids can do as well as you--or better than you. That's a lesson everyone should enjoy learning--well, almost everyone.