I say if your gone for a weekend then leaving enough food and water is enough. However, 3 or more days consult a boarding place or petsitter. Your vet may also do boarding. Mine does and is cheap.
I just recently pet sitted. I found that the cats did not eat for a day after their owner's were gone. It was the "stress" of them not being around that they didn't eat much the first day. They were also "less active" during that period as well.
I've left my dogs for a weekend before and have returned to food still in their bowls! They wouldn't eat much until I got back. Unfornately, when Scooby had his "stroke" a few years back. He wouldn't go to the bathroom unless I took him. He was at the vet's office for a week for treatment/observation. I had to go there every day they were open and walk him myself to get him to go! Unfornately, I wasn't aware of this fact until he wet himself in the vet's visitation room and fell in it!
So I say, make sure the person you leave the animal with is familiar with the animal prior to the pet sitting. Let them walk the animal alone. That way the dog will fill comfortable with them. Otherwise, I say enjoy your weekend and let them enjoy theirs from taking care of you all the time!