Dalmau Park is a lovely spot to take a breather. Have a picnic, throw a ball around or just lie on the soft grass and close your eyes for a while.
You're spoiled for sightseeing choice when staying in Calella. Dalmau Park is a great place to begin here, but there are plenty of other fine spots to experience. Marineland, Levante Beach, and Malgrat de Mar Beach are not far from the recreation and green space and will fill in your day nicely.