You could use hooked or braided rugs. Bright with strong contrasts. Strong colors go well with the natural-looking textures of informal furnishings. The size of the design should fit proportionately with the size of the room and its furnishings, but on the whole, small motifs seem more informal. Uneven or un-shiny.
Texture is a great determinant of informality.
Textures in an informal room tend to be natural and unpolished. They tend to have uneven surfaces, or look like they have uneven surfaces. A carpeting colored with various shades does not look uniform. Mottled colorations or tweedy effects even on smooth surfaces give an informal ambience. If bright, bold colors are informal, and pale, subtle colors are formal, then those colors in the middle are semi-formal.