Napa is a short drive from Fresno and a fun treat when you want to escape heat, have fun, enjoy some luxury, and also get some hiking done. One great location in the Napa area for hiking is Robert Louis Stevenson State Park's Mount St Helena hiking trails. The Mount St Helena Trails begin a few hundred feet from the parking area and are very very well marked.
You begin this hiking trail at a gentle grade so you can get a nice warm up. The hiking trail weaves through the forest and brush for just under a mile. After hiking ust under a mile you reach the monument commemorating Robert Louis Stevenson's honeymoon. At this site on the hiking trail he spent his honeymoon camped out in an abandoned building. Right after you reach this monument the hiking trail gets steeper. You will finish your finish forest hiking section and begin hiking across a fire road. Your hiking journey levels out and you have an amazing view of Napa Valley, Mount Diablo, and Mount Tamalpais. You then continue hiking on the southern slope of the mountain and get additional amazing views. At just under 4 miles you reach a fork. You can turn back or take the fork to the left and begin the South Peak hiking trail which is approximately half a mile.
Now for some hiking details. A great thing about the hiking trail is that there is no admission fee. However the drawback is that although in my experience the trail is well marked in most areas a significant amount of it is a bit overgrown or on a fire road. If you want only dirt hiking trails you may prefer the nearby Table Rock, Oat Hill Mine, or Palisades trail instead. Additionally if you do not want to do elevation hiking you can hike in the meadows area. About 2.5 miles through the meadows at the base of the trail there are some beautiful views of redwood benches and trees. Another thing to consider is that there is no camping and also there are no bathrooms so you may want to arrive early with supplies to pack out all of your trash and other refuge.