Roughly in the centre of Fish Creek Park is Votier's Flats, a picnic area and trailhead for the numerous trails that snake their way through the park.
Near the parking lot is a washroom and some picnic tables in full sun, you'll need to either explore to the south or west to find some picnic tables which are shaded. Some of the picnic tables have fire pits nearby if you want to roast marshmallows or cook some food.
Curving around the flats is Fish Creek, which offers a decent swimming hole to the west or any other water sports you can do in a shallow creek.