Alki would be it. Alki is one of only a few sandy Puget Sound beaches in Seattle, which makes it hospitable to sunbathing, vollyball, and generally lots of fun-seeking folks.
Sand: the sand seems safe to walk around barefoot in... not much broken glass or anything like that. However, the sand is quite dirty from people making (illigal) fires on the beach and leaveing their charred wood to dispurse into the sand. So expect dirty feet.
Water: Puget sound is cold! But if you're thick skinned, this is a great swimming beach with a gradual slope and a sandy/pebbly substrate into the water. There is occationally big clumps of seaweed that wash up, but nothing too alarming.
Bike/Rollarblade/walking path: there is a nice scenic paved path right along the beach that is busy with people strolling, exerciseing, rideing rented adult-tricycles, etc.
The scene: lots of all types of people! hardbodies playing vollyball, familys having picnics, kids running and splashing around, teenagers hanging out with friends, old folks exerciseing on the path.
Beachfires: are allowed in the designated fire pits, but not out on the beach.
Services: there are public bathrooms, and a ton of restaurants/coffee shops/ice cream shops, rollarblade rentals, etc just accross the street
Parking: can be tough. there is street parking all along the beach, but it fills up quickly and you may be forced into the surrounding neighborhoods on a busy (sunny) day.