I claim the ornamental design for a stackable tray, as shown and described.
FIG. 1 is a perspective view of a stackable tray, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right-side elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a bottom perspective view thereof.
The broken lines showing a glass of wine and food in FIG. 1 are included for the purpose of illustrating the invention in use, and form no part of the claimed design.