Spend it out by Lake Hefner! The site of the Redman Triathlon in September, the Lake Hefner trails are hard-surfaced and suitable for running or biking. You can ride your bike up on top of the Hefner dam for a great training ride, and then head out on the Redman bike course out of the city for longer rides.
If you like to mountain bike, right by Lake Hefner is Bluff Creek Park for the best singletrack mountain biking in Oklahoma City. Although there's only about 4-miles of trail, it's twisty and turny, and you can make some nice loops out of it. There's also a hard-surface trail in the park for roller blading or running.
The perfect way to end the day would be to eat dinner at one of the restaurants right over Lake Hefner - the sunset will be spectacular.
Any direction out of Oklahoma City will lead to some unexpected gems. Head south on I-35 to the Arbuckle Mountains to visit Chickasaw National Recreation Area for camping and hiking by some unexpectedly cold and clear springs. If it's a warm day, you can go swimming at the national park. Near Chickasaw is Turner Falls Park, which is a little pricier and commercialized.