I recommend riding a bike through the Reserva until you arrive at the Plata River (the Costanera, as it's called), where you can relax and have a picnic. Or once you've exited the Reserva, stop by a food truck for a choripan or bondiola, two of the most popular (and delicious) local sandwiches.
The ecological reserve has some good running paths away from the urban areas, but sometimes its not open.
This is an excellent place for a walk in the city. There are well maintained paths and display boards with clear maps of the trails and examples of the animals that you might see. The large pools support water fowl and this enables a good range of birds to be observed. It was a hot day when we visited but there are plenty of trees to provide shade and places to sit and have a picnic.