Hiking can be an amazing and cheap way to get some exercise. If you want a relatively out back trail and is not too crowded in Yosemite national Park Poopenaut Valley is just the key.
The trail head is approximately 70 miles from Fresno. The Poopenaut Valley trail is only 2.1 miles round trip. However it is a bit challenging due to the approximately 1,300 foot elevation gain and out back type trail maintenance. Additionally it is an inverted hiking trail. Therefore you are hiking down hill for over 1,00 feet on the start of the hiking trail. The end of the trail involves climbing up. Perhaps this is why the word "poop" is in the name of the trail. This makes for a great exercise because it really forces you to push yourself.
There is a beautiful lake on the trail. It looks amazing however I would not recommend swimming in the lake. It is extraordinarily cold, even in summer because it is fed with High Sierra snow melt.
There are amazing view while hiking, particularly in the Fall and Summer. You reach the trail head from Highway 120 by entering Yosemite from the east and going north on Evergreen Road. You then continue for just over 7 miles and turn right onto Hetch Hetchy Road. You follow this road for over 5 miles past the ranger station to the sign "Poopenaut Valley". You park on the side of the road and start at the trail head heading downward.