San Luis Creek runs through the middle of downtown, and Mission Plaza is the park that grew up along it directly below the Mission. It's a genuinely pleasant patch of green: big trees, benches along the water, the creek running shallow and clear enough that you can see the bottom. On warm afternoons people eat lunch here, and during the Thursday Night Farmers Market the plaza is full from mid-evening on.
The layout puts the Mission above on one side and the San Luis Obispo Museum of Art on the other, so Mission Plaza is the natural hub of a short downtown circuit. The creek walk extends a few blocks in either direction from the plaza and is a nice way to move through the center of town without using the streets. Footbridges cross at intervals. It's not spectacular, but it's the kind of small public space that makes a downtown work, and SLO is better for having it.
In spring and early summer, sycamores along the creek leaf out fully and the shade is deep. In winter the creek runs higher and faster after rain. If you've come to understand what makes SLO pleasant rather than just convenient, this is a good place to sit for twenty minutes.