Just them.

I am always looking for simple and fun (plus cheap) activities to do with the kids. Making smores in the backyard is one of our favorites! You can buy a cheap "fire" pit- they sell them everywhere and you don't need "fancy".
Graham crackers, marshmallows and chocolate! Put chairs around the fire and woila! A perfect summer activity! It's also a great time for the family just to sit together and talk. We had a great time listening to my son's camp stories and hearing about all the things he did.
I am always tempted to invite friends over for the fun. But, as I look at my kids, I realize they are growing up really FAST! I have become a little more "protective" of my time with just them. The greatest and lasting relationships that they will have will be "family".