I bought a headboard that I'd like to just stain darker without going through the whole stripping/sanding process. Is there a product or method that is best for this?
You can use "furniture glaze" (pretty much a tinted "clear" finish.)
You could tint/dye a clear finish, or use a naturally tinted "clear" finish such as dark shellac.
For both of those you'd likely need to at least scuff-sand the current finish to get good adhesion.
You can also use a colored/tinted furniture wax over the current finish.