Once you're in orbit, you're halfway to anywhere. The moon, Venus and asteroids are all easier targets than Mars. Some of the moons of Jupiter may well be easier targets too. Mars is hard because it's big, with a thin atmosphere, so you need heavy equipment to land and take off. For Venus you just stay in orbit. For smaller moons with no atmospheres you need a lot less mass in fuel and heat shielding to land and take off again. You can use that space for more life support and radiation shielding.