I had several guinea pigs that loved being outdoors in the summer, so my mother and I set up a little enclosure around some tall grass. They loved it! Unfortunately, the little area we set up for them could be escaped from way too easily and we would go inside for a couple minutes and find that some guinea pigs were missing.
We recovered a couple of them, but I can't possibly stress the importance of a solid playpen! Make sure to watch your rats diligently when they're outdoors, even in the playpen.