Swanbourne Beach is an iconic location, which attracts a more local crowd than the nearby Cottesloe Beach. As all Perth beaches, Swanbourne is generally calmest when an easterly wind is blowing, which is generally in the mornings (be sure to check the wind forecast).
There is plenty to do here - take a walk on the beach, body surf, exercise your dog (dog beach is to the south) or just enjoy Perth's beautiful weather by laying on the beach. Be careful with little ones though, there can be dumping waves close to shore.
The easiest way to get there is by car, with a decent car park behind the beach. Between the car park and the beach is great coffee at a hole in the wall shop - "No Coffee for You," and if you are looking for a sit down breakfast, lunch or afternoon drink check out Shorehouse. The "Swanny Surf Club Brekky Dish" is fantastic!