What can I do in yosemite national park for 6 hours?

Have you looked at a map? It may help you with this question. Are you driving into one part of the park and leaving thru another part? There's a fair amount of driving in the park. The Yosemite Valley is the most famous and most scenic part. Be sure to drive into the valley. There are various Visitor's Centers, which are like museums. If you have time you could do a short hike. Check the national park service web sites.