A day trip through Yosemite would be way better than Sequoia. Fly to Mariposa, drive into the valley on 140 then take the southern route out through Wawona where Mariposa Grove is. Plenty of big and famous redwoods to see there. If you simply must go to Sequoia then you must take Hwy 180 at the junction and go all the way down into Kings Canyon. If you're that close you really shouldn't miss it.
Last I heard, there were no rental cars at Mariposa anymore.