Grand Street Bike Trail

If you start at SW 15th and Portland location, it has a decent uphill climb going to the South, once you reach 29th Street and start heading west it really levels out nicely.  The start of the path is on the Oklahoma River Trail, take it east for a bit and you'll see a sign to your right saying Grand Street Trail---->  from there in becomes sidewalk for the trip south and then back to a bike path/larger sidewalk to the east and back north.  You then finish your trip back to the start using the Oklahoma River Trail going west again.