Filters are great for preparing photos for mixed-media compositions
Use the Filters Photocopy or Find Edges to get quick outlines. When using Find Edges, you may need to desaturate your image (Cmd/Ctrl+Shift+U) if the result is too colourful.
For an illustrative vibe, you could use Filter like Colored Pencil, Crosshatch or Graphic Pen under Sketch.
To get a vector look, try using Cutout under Artistic, using the Filter Gallery as a laboratory to test out endless combinations!