A plain bathroom mirror without a frame may be extremely useful, but also extremely uninteresting. Replacing that mirror with one more stylized might not be an option, especially if the mirror is permanently affixed to the wall. Instead, embellish the existing mirror to give it that style it's missing, using inexpensive craft supplies or collected materials.
Glass Gems and Pebbles
Flat glass gems, commonly used in the bottoms of clear vases as decor in a flower arrangement, dress up a plain wall mirror, giving it a more custom appearance. These gems can be clear or in shades such as green, aqua or cobalt blue; use one shade for a uniform appearance, or mix up several for a more random look. A dab of hot glue on the back of each gem holds it in place around the perimeter of the mirror. Just lay out the design on the mirror, then lift each glass gem to glue it down. If the mirror cannot be removed from the wall, strips of painter's tape looped onto the wall sticky side out hold the glass layout in place until you are ready for glue. Polished river pebbles can be applied in the same fashion.
Glass or ceramic tiles, 1 inch or less across, turn that plain mirror into a decorative statement piece for the bathroom. Rows of tiles already set on a backing mesh are available at home improvement stores; just cut those into a width that looks good against the bathroom mirror, then adhere them with silicone sealant or construction adhesive. Glass tiles in light blue, green or sand tones add a seaside vibe to the mirror, whereas dark shades may give the room a more neutral feel. Apply grout between tiles for a finished look.
Creating a mosaic mirror is similar to applying tiles, but instead, you can use bits of broken tiles, sea glass or broken china. Clear silicone sealant holds the broken bits on the mirror and stays translucent, which is useful for sea-glass or clear-glass mosaics. Grout between the pieces once the silicone dries creates visual space between each piece. Small plastic toys such as gumball-machine prizes or action figures can be used for a quirky mosaic frame suitable for a child's bathroom.
If you wish that unframed mirror actually had a frame, create one with decorative molding, or reuse planks of wood from an old shipping pallet or scrap wood. To get a finished look, cut the edges of each piece of trim at a 45-degree angle; some home improvement stores will do this for you. Construction adhesive holds each piece of wood up on the mirror, arranged like the pieces of wood on a picture frame. Painter's tape holds pieces in place while the adhesive dries.