What about Europa? Sure, Jupiter is long way out, but Europe has a ready supply of water, and Jupiter's moon system probably has useful in-situ resources we could use to manufacture fuel. Then you have a gas station on the way to the outer solar system.

Realistically, the Moon is probably the first place to go, since it's close enough to allow easy resupply from Earth. If we embrace nuclear propulsion we should be able to make fairly regular trips between Earth and Mars.