An enclosure for enclosing an area including a frame and a canopy assembly. The frame includes an upper laterally-extending support, a first upstanding support coupled to a first end of the upper support, and a second upstanding support coupled to a second end of the upper support. The upper support is length-adjustable. The canopy assembly is connectable to the frame for defining an enclosed area, and includes a plurality of collars removably connectable to the frame, and at least one panel holdable in a plane defined by the supports of the frame. The circumference of at least one such collar is adjustable so that when the length of the upper support is adjusted, the collar circumference can be adjusted to accommodate the change in length.