"The Case for Mars: The Plan to Settle the Red Planet and Why We Must" by Robert Zubrin actually describes a really simple way of providing gravity to whatever strength you desire.
Paraphrasing somewhat, you have a habitat/capsule/call itwhatyouwill atop the third or fourth transmars injection stage of what is essentially an upgunned Saturn 5 type rocket.
The rocket burns to push the spacecraft into a Mars trajectory (or wherever it is you're going)
Once the burn is complete and whole ensemble is on the 6-month coast to Mars, instead of completely jettisoning the stage, it is winched out on a tether as the spacecraft does a small burn to push itself away.
Once the end of the tether is reached or the desired length has been paid out, small thruster rockets are fired on both the spacecraft and the stage, so that they spin about the common axis/centre of gravity. This will impart a G-force/centrifugal force on both the the spacecraft and the stage, the magnitude of which will depend on the radius and speed of rotation, which will depend on the relative mass of the two units.
The stage could still have fuel in it to use at Mars, in which case you could simply winch the two units back together and use thrusters to stop the spin, or if not, simply cut the tether when the spacecraft is correctly positioned with respect to its trajectory.
You can vary the speed of rotation or length of tether to vary the gravity, so pretty simple task to gradually boot it back up from Mars (0.38G) to Earth (1G) on the return trip. That way, everyone hopefully returns with the same bone mass as they set out with.
The thing I like about this solution is that it's simple, elegant and cheap - doesn't require anyone to reinvent the wheel, (because basically, it is a wheel).
