The ornamental design for a bag for a playpen, as shown and described.
FIG. 1 is a perspective view of a bag for a playpen, showing my new design, the broken lines are for environmental purposes only and form no part of the claimed design;
FIG. 2 is a side view of the bag for a playpen as shown in FIG. 1;
FIG. 3 is an opposite side of the bag for a playpen as shown in FIG. 1;
FIG. 4 is an end view of the bag for a playpen; and,
FIG. 5 is a bottom plan view of the bag for a playpen as shown in FIG. 1.