I claim the ornamental design for a stimulation device, as shown and described.
FIG. 1 is a perspective view of a stimulation device, showing my new design.
FIG. 2 is a rear of the stimulation device.
FIG. 3 is a front view of the stimulation device.
FIG. 4 is a right side view of the stimulation device.
FIG. 5 is a left side view of the stimulation device.
FIG. 6 is a bottom view of the stimulation device; and,
FIG. 7 is a top view of the stimulation device.
The logo and control buttons shown in broken lines are for the purpose of illustrating environmental structure, which form no part of the claimed design. The remaining broken lines are for the purpose of illustrating portions of the stimulation device, which form no part of the claimed design.