The ornamental design for the activation energy serum small bottle, as shown and described.
FIG. 1 is a front view of an activation energy serum small bottle of my design.
FIG. 2 is a rear view thereof.
FIG. 3 is a right view thereof.
FIG. 4 is a left view thereof.
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the activation energy serum small bottle and form no part of the claimed design.