For my sons birthday, i set out and arts and crafts table for some of the children to keep them entertained. i set out markers, glue, construction paper, and some other stuff, and just let them have fun creating.
it depends alot on the age group of children you are inviting as well..
its summertime, and there are lots of bucket and shovels at inexpensive places like the dollar tree and 99 cent store that you could maybe make like construction favor buckets??! you can also pick up some plastic dump trucks, and various constrution vehicles there as well. maybe make a few center pieces with those and soem balloons!
hope this helps!