Life on Earth started in the oceans, why not repeat the process. Fluorinated hydrocarbon liquid is 'inhaled' by deep sea divers because it can hold alot of oxygen, and some carbon dioxide, simulates the pressure of deep water, and is otherwise biologically inert. Fill a crater with the stuff, cover it with a layer of immiscible hydrocarbon liquid that is less dense, does not transport oxygen, but transmits sunlight. Wham! You've got a stable ocean on Mars that can support an ecosystem.... Or at least an aquarium. Start with your basics: bugs, plants, fish. Work up from there. It's cool so long as you can manufacture the stuff using Mars resources.