I use the foam playmats in my playhouse that is outside since there is no floor in it and keeps the kids out of the mud that happens when rain gets in the window and door area but the sun doesn't. We left them out all year and only took them out a couple times so we could pull up the grass that grew between them and the house. They were still fine at the end of the season and we just washed them down and stored them for the second summer. This will be the third one and they seem to be in ok shape still.
I have done playpens outside, a play yard on the patio with a quilt on the bottom of it. Some babies do well in a plastic swimming pool with some toys.
Going outside as soon as the babies wake from morning naps means I can put the older kids out to play - the door is off our kitchen and have the babies in highchairs having snack either just inside the kitchen with the door open or moving the chairs to the patio. With a dish of cheerios they will take awhile to go through them. And the good news is the squirrels do any cleanup necessary.