It might just be me, but I felt like Kings Canyon was the best place for me to get in shape. It's away from the crowds of Yosemite and empty enough that no one will watch you sweating and panting as you hike through the beautiful redwoods. It's a beautiful place, and the weather in the summer is always fantastic. I'd combine a trip here with a tour around northern California. I spent three weeks hiking, shopping and relaxing and I lost two dress sizes without altering my diet! Finally, the best point about the national parks is that they're very family orientated so there will be something for everyone.
www.nps.gov/seki/index.htm
83918 Grant Grove Dr, Kings Cyn Nat Pk, CA, United States
+1 559-335-2856
Google map: bit.ly/sk8E7C