What month will you be in Sacramento? How far are you willing to travel out of the downtown area? And what is "best, coolest and most important" to you? I might be able to help a little with more info. Some areas for you to consider :

Nature

Architecture

History

Outdoor activities/sports, etc.

Shopping

Adrenaline/adventure stuff

Dining

Art/Museums/Theater

Based on your other posts here, it looks like you have 12 days in CA to see the sites. If that's all you have then you might want to consider skipping Sacramento for areas with a bit more interest to you especially if your sist will be during the summer when the temperature is quite hot.

CA is a huge state with a variety of things to see. Consider Tahoe, Yosemite, Sequoia, Mammoth Lakes area, the coast etc.

The coolest - rent a raft & drift down the American River.

Most important - visit the State Capitol bldg. & adjacent park.

Best - California State Railroad Museum & adjacent Old Town.

Don't come here if you only have 12 days. There's many better tourist destinations in CA then here.

Suiters Fort is worth a couple of hours at least.

My two favorites in Sacramento

Sutter's Fort

http://www.parks.ca.gov/?page_id=485

Railroad Museum

http://www.parks.ca.gov/?page_id=668

Personally, I wouldn't skip Sacramento altogether like others have suggested.

I would, however, make it just a half day trip on your way from SF to Lake Tahoe.

Get here in the early afternoon and see the State Capitol, Railroad Museum and Old Sac (http://www.oldsacramento.com/)

Stay at the Delta King (a turn of the century sternwheeler permanently moored in Old Sac as a hotel and restaraunt) or stay at the Embassy suites. Then head up to Tahoe the next morning.

TJ

I would say the State Capital and the railroad museum. At the railroad museum you will be in Old Sacramento, with great restaurants and fun shops to walk around. don't worry about it being hot, if you do these things they are inside. Walking around Old Sacramento, its mostly shaded. Have fun.