A playpen is a great way to entertain your baby in a safe environment. Many modern playpens have mesh sides and come with a safety seal of approval. Do a proper once-over every few days to make sure the small mesh holes aren't getting larger, and keep an eye out for things like exposed staples and loose elastic. Don't leave a lot of large stuffed toys lying around in it either. Resourceful babies have been known to stand on things and climb out when unsupervised, which is a no-no to begin with.