One of my son's friends gave him his phone number one day. I called it and talked to the child's mother and we set up a play date. With another child in my son's class, the mother sent a note with him to school and asked the teacher to send the note home to me. The note asked for a play date and included their phone number.
You find ways to get to know your child's friend's parents. I personally really liked that one of the parents sent a note home to me. It was very nice and when we went over there the kids had a blast and so did we (the parents that is).
I was one to have small little lunches, or snacks, juices handy, and some toys all over for any children. When it's calm.
Other days out are park days,or walks, or going to malls. Or just food shop. Always take the children if you can.
Like before on the first line here, keep plenty of toys, and have a snack stockpile. IT's good for repoire. It makes everyone feel included.