What are some good 1 day trips in California for sightseeing and fun?

Yeah the "middle of California" can cover 400 miles. Sacramento is much different than say Fresno. I'll ume you live near Sacramento. Lake Tahoe is only a few hours away and is beautiful. You could also head over to Napa Valley and see the wine vineyards. Len National Forest is beautiful and it is actually relatively easy to hike to the summit of Len. Lake Shasta and Mt Shasta are also a few hours North. That's pretty much the best I can do without you being specific.