I really like marrow the way that theonlyone suggests, but you can also serve it warm with a dressing, for example a lemon-and-pepper-based one, or a rosemary jelly, or raspberry dressing (all available from big supermarkets).
or
you could simmer it in a large-based pan with vegetable or meat stock (cubes are fine) and season to your taste. My gran used to add pieces of bacon and passata and small onion cubes which was not time-consuming at all but very tasty. The only problem with marrow is getting it to the texture that you like, as some people like it really soggy but most people like it still quite firm