Kasinonranta is Lauttasaari's easy summer beach, with a playground, lawns for picnics, and a seasonal cafe nearby. It is more of a local family beach than a headline sight, but it is a good warm-weather break if you are staying west of the centre.
Bring a towel, expect Baltic water temperatures, and use it as part of a relaxed Lauttasaari walk rather than a full-day beach trip.