Hi,
Welcome to the forum. I think 1 1/2 - 2 days would be plenty of time to see Sacto. But it all depends on how long you like to linger places. You could probably spend an entire day in Old Sac itself if you wanted.
In Old Sac you have the RR Museum, which you know about. But there is also a 1800's schoolhouse, a paddlewheel riverboat called the Delta King (now a hotel/restaurant/theater), a small Wells Fargo museum etc. You can ride a steam train along the river too. Oh... FYI, Old Sac was the western terminous of both the Pony Express and the Transcontinental Railroad.
There is a brand new shop in Old Sac across from the RR Museum called 'Trailmix'. The kids would love it. It has things for taking kids outdoors and discovering. Outdoor education stuff. Their sign hasn't been approved yet but it's right next to the Salon. Download their Old Sac Scavenger Hunt. http://www.oldsacramentoscavengerhunt.com/
I think they have one for Yosemite too.
Another toy store the kids, and adults, will like is G. Whillikers. It's on Front street just south of K street.
Regarding times;
RR Museum I would say takes around 2+/- hours.
Old Sac, 2-3 hours.
The Capitol, around an hour.
The Governors Mansion, under an hour. (the Stanford Mansion is equally as nice or nicer)
Haven't been to the Discovery Museum you're talking about so can't comment there. The one that is in Old sac is no longer called Discovery Museum.
You said "I've heard that you can spend a whole day at Sutter's". Are you talking about Sutter's Fort in Sacramento- or Sutter's Mill where gold was discovered in the town of Coloma, aka Marshall Gold Discovery Site- or are you talking about the town of Sutters Creek?
Sutters Fort is in Midtown Sacramento not too far from the Capitol etc. If you want to see this it would take a couple hours and should be done early in the day as it will get hot in the afternoon. http://www.parks.ca.gov/?page_id=485
Coloma/ Marshall Gold Discovery SHP is about an hour and half out of Sac. and would take at least a couple hours to see. www.parks.ca.gov/default.asp?page_id=484
Sutters Creek is a small Gold Town about an hour south of Coloma on Hwy.49. I'm not aware of kids things to do there but it is a cute little town that warrants an hour or two of window shopping. If you are there around lunch time go to Thomi's at the north end of town, great Deli! Get the BLAST! Bacon, Lettuce, Avacado, ? and Tomato mmmm.
Another hour south just off Hwy.49 is the town of Columbia. It is also a state historic park and definitely worth a visit. They have stagecoach rides and lots of shops etc. see http://www.parks.ca.gov/?page_id=552
There also are mine tours that you could take near both Sutters Creek and Columbia.
Once you get to Columbia you are within a couple hours drive of Yosemite.
Have Fun!
TJ
