Methane and the other ethanes; crack 'em down for O2, sure.
Ammonia is too valuable as fertilizer; ship it down in bulk for farming.
And, of course, plant cultures of blue-green algae *everywhere*; the stuff will actually grow in Mars' current atmosphere, given a bit of water, and convert CO2 to O2 while it does.
Shipping CO2 from Venus to Mars will allow for increasing the atmospheric density fairly quickly.
Ammonia is too valuable as fertilizer; ship it down in bulk for farming.
And, of course, plant cultures of blue-green algae *everywhere*; the stuff will actually grow in Mars' current atmosphere, given a bit of water, and convert CO2 to O2 while it does.
Shipping CO2 from Venus to Mars will allow for increasing the atmospheric density fairly quickly.