Consider the International Space Station, that marvel of incremental engineering. It has close to 15,000 cubic feet of livable space; 10 modules, or living and working areas; a Canadian robot arm that can repair the station from outside; and the capacity to keep five astronauts (including the occasional wealthy rubbernecking space tourist) in good health for long periods. It has gleaming, underused laboratories; its bathroom is fully repaired; and its exercycle is ready for vigorous mandatory workouts.
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Send the ISS somewhere.
The ISS, you see, is already an interplanetary spacecraft -- at least potentially. It's missing a drive system and a steerage module, but those are technicalities. Although it's ungainly in appearance, it's designed to be boosted periodically to a higher altitude by a shuttle, a Russian Soyuz or one of the upcoming new Constellation program Orion spacecraft. It could fairly easily be retrofitted for operations beyond low-Earth orbit. In principle, we could fly it almost anywhere within the inner solar system -- to any place where it could still receive enough solar power to keep all its systems running.
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Let's begin the process of turning the ISS from an Earth-orbiting caterpillar into an interplanetary butterfly.
