I've flown a significant number of hours in Cessna 210s. Expect it to be loud - I suggest bringing earplugs. Try not to sit in the rear two seats - they are really more for children than adults if you ask me. The size of the cabin will be similar to a car. The ride will really depend on the weather that day - hanging out below 12,000 feet you are definitely subject to thermals and turbulence caused by wind blowing over mountains and such. Some days will be completely smooth, other days not so much. With about 600 hours flying small planes, the turbulence doesn't bother me - and it's really no worse than driving down your average highway - but my wife usually takes Dramamine when she flies with me because she doesn't handle the turbulence as well. It wouldn't hurt to take some an hour before your flight if you think it'll be a problem.

Also, don't have any worries about safety. The plane is quite capable, and the pilots will not fly in conditions that are unsafe. For the most part, any outfit that is operating as an airline is subject to all the same safety regulations, whether they fly Cessnas or Boeings.