They got to Mars in the first place.
The rovers are there, have been there for decades. Relative non-challenge to send a rover to Mars, or any other solar system object. If you want to expand the space program, you will have to simplify the systems that get you there in the first place. If the ISS costs tens of billions of dollars just to keep 3 astronauts alive in low-earth orbit, then the resources needed to do other things won't be available. Dump the astronauts, and everything gets easier. And cheaper. And more near-term.