Three of the major problems facing a long-duration Mars mission are high-radiation cosmic rays, solar flare particles, and degradation of astronauts' physiology due to low gravity.
Here's a solution that solves all three of these with a single approach.
First, if the space ship is tethered to a counter-balance and rotated in a large arc, it created centrifugal force with a minimum of material and weight.
Second, if a circle of lightweight conductive material is extended just beyond the arc of the rotating ship/counter-balance assembly, it will act as a standard generator as the ship and counter-balance act as positive and negative poles sweeping past the light weight conductive circle. This power can be used to create a magnetic field that would deflect cosmic rays.
Third, in chill of space, use of the proper materials in the large circle construction could create a super conductor effect which could multiply the power generated so that it would become strong enough to also deflect solar particles.
Just a thought.
Here's one more thought. Addressing only the gravity issue, experiments are being done by NASA at Langley using small-scale centrifuges to create individual high gravity simulation environments for astronauts. But, severe disorientation occurs, and is blamed on the speed of rotation necessary to generate enough "gravity" in a small system.
The reason this disorientation occurs in small scale centrifuge systems is actually due to situation that the head of the subject, being near to the hub of the rotation, is getting very little gravity boost due to the slower tangential speed, while the lower body at the outside is receiving near-normal "gravity".
The brain cannot parse the differential between the near zero gravity of the head and the near full gravity of the feet as it has no evolutionary experience with such a situation. To test my hypothesis, a subject could be laid out on his or her back along the arc of the rotation so that gravity is nearly identical from head to foot. This could supply a full-gravity therapy during the entire daily sleep cycle in a compact space for the entire mission. This might be enough to prevent any loss of muscle or bone calcium due to the waking hour zero-gravity conditions.
In practice, a second astronaut could sleep on the opposite side of the system at the same time, providing a counter-balance for smooth rotation thereby minimizing the necessary structural mass of the mechanical system itself.
Again, just a thought.