I have never really used a playpen or pack'n'play for its intended purpose of corraling a child. I'm not sure why, just never seemed like that would go very well with my independent and opinionated children. BUT, I did see this idea somewhere (maybe in an issue of Family Fun?) to use the playpen in reverse, and have loved it. Keeps little brother out of sister's way and all the screeching of "No, Wyatt, no no no!" to a minimum.
Another idea we tried (probably also from Family Fun) before all the sickness hit was this contained paint idea. Simple: Put paint in bag. Seal up tight. Add tape for extra security. Give to kids. Sit down and enjoy the peace for a few blessed minutes.