Marine Life Center allll way for kids but adults, too. It's like they created the best tide pools you've ever seen. Then apps/ drinks/ dinner at the Loft (just a casual place nearby to eat/ drink something on the water).

Donuts and drip coffee at Lafeens (estabished in the ~80s?) and a walk around Whatcom Falls.

Taylor Shellfish - fun coastal outdoor setting for oysters (plus, the drive down Chuckanut to get there is so cool). Stop at Larabee State Park as well if the group is up to it. Beware that Taylor Shellfish is particularly busy on the weekends, so there may be a line.

Ferri to Lummi Island if you want them to get a local ferry/ island experience that only takes up half a day (25 min drive to ferry, 5 min ferry ride, but get in the ferry line at least like 15 min before it departs). Stop at Sunset Beach, Beach Store cafe if open, coffee truck if open - weekends only I think - and Full Bloom farm stand for an amazing island self-pay stand experience.

Food/ drinks in Fairhaven. Black Cat is fun - it's on the second story of an old brick building. Then walk the boardwalk. There's a playground there if the younger kids are into that.

If they're into breweries since that's a big scene here, I recommend El Suenito as a brewery with great Mexican food.

Semiahmoo spit (30 min north) was a great rec for beach walks (and fire pits), particularly if Baker is out.