We spend nine months in the womb, so why not fill the spacecraft with water.
It sounds crazy at first, but water offers radiation protection including thermal protection, we can drink it, it makes good use of the empty space AND if the spacecraft is rotated slowly the centripetal acceleration of the water on the human body should mimic gravity.
A 10m water column on earth is approximately two atmospheres, and long durations breathing at this pressure may require decompression however an appropriate sized water column slowly rotating in space should be exactly one atmosphere - no problem! It should mean that calcium won't leach from our bones, our bones won't pit or become brittle as is the normal case with pressurisation and microgravity, and we have viscous water, and artificial gravity to exercise against meaning our muscles won't waste away.
Sure we need to make the spacecraft instrumentation waterproof, not that difficult, and we need to be able to breathe, but we could make an external gill or something like cave dwelling axelotyl, or make do with a rebreather. On Mars, same system just take refuge at the appropriate depth in the bottom of a watercolumn, and you won't need to spin it but will receive the same benefits! Let's try it- email firstname.lastname@example.org. On EVA consider a water presurised drysuit as the basis of a spacesuit. Put funky display units inside the helmets. Not all the spacecraft needs to be filled with water, have a submarine inspired airlock. Finish the job and include a Polywell derived IEC fusion and plasma propulsion system (that will get us to Mars in six weeks or less with a ionocraft inspired external hull for Earth based atmospheric propulsion).