Collage is probably the most well-known type of mixed media, because it has so many applications and so many materials can be used to create it. Paper collage may involve smaller works of art worked into a larger work, photos, pieces of tissue, vellum or found objects. The construction involves an adhesive, which can be anything from archival paste to clear tape. Pieces may overlap, or the layout may be clean and minimalist, depending on the artist's style.
Mixed media paintings on paper may start as one type of painting---watercolor, for example, or acrylic---and then they are built up with one or more other types of paint that aren't traditionally used with the type of paint used for the base. An artist may paint a light-colored abstract background in watercolor, then go in with oil paints to add dark details that seem to pop off of the paper.
Printmaking allows an artist to layer different types of prints into a one-of-a-kind finished product. For example, a silk screen design can be printed on paper 100 times, and the artist can then lay down a layer of wood block printing over them, making each one a unique piece. Prints can also be embellished with hand painting or drawing, or cut apart and reconstructed into new designs. Even photo processes can be applied directly to paper prints to create a mix of traditional printmaking and photography.
Pen and ink drawing lends itself to mixed media, because it's generally done in black and white only. Colors can be added using watercolors, acrylics or oils. Whether the colored paint is applied first or added after the ink drawing is made depends on the look the artist wants to achieve and the type of paint used. Watercolor is translucent enough that you can paint over ink and have it show through, while more opaque paint, such as acrylic, will cover the ink lines.