Depending on the personalities of the children, play dates can go extremely well or the exact opposite. It's always best to have a little structure when you are the play date host. This can help you avoid the nightmare scenario. Here's the beginning of a list of suggestions  I will compile over time.
* Do a dress up play date where you have several old clothes on hand. The children can choose their favorite dresses, pants, hats etc. This is something that will occupy their time and keep them focused.
* As long as they keep the colors on the paper, coloring with crayons can keep them going for a decent amount of time. Obviously, a little supervision in the beginning is advised depending on the attention span of the children.
*Dancing is a good possibility. Of course, with age appropriate music this could be quite fun. MP3 player, CD player, DVD's, what have you, can be used.
*Picnic playdates can be quite fun for both the children and the parents, so grab a basket, picnic blanket, and make a day of it.
*A playdate to a children's museum could be a good way for parents and kids to hang out, while enjoying a meaningful activity. Of course, depending on how many kids are on the date, you would want to match that amount of adults with kids on the playdate for supervision.