Kids of all ages love Birch Bay Water Slides (4874 Birch Bay-Lynden Rd., 360/371-7500, www.birchbaywaterslides.net [1], daily mid-May to Labor Day, $12 ages 6 and up, $8 ages 3–5 and seniors, free for tots). The park has a 60-foot “hydrocliff," four giant water slides, lots of other kiddie slides, plus a volleyball court and hot tub for oldsters.
Pioneer Valley Railroad (4620 Birch Bay-Lynden Rd., 360/371-7700) has miniature train rides and go-karts in the summer.
Renaissance Adventure Guides (866/424-8433, www.raguides.com [2]) leads sea-kayaking trips from Lummi Island [3], from half-day local paddles to three-day trips through the San Juan Islands [4].
Peace Arch Equestrian Center (360/366-4049, www.peacearchequestrian.net [5]) has horse and pony rides near the outlet shops in Blaine.
Links:
[1] http://www.birchbaywaterslides.net
[2] http://www.raguides.com
[3] http://www.moon.com/destinations/washington/north-puget-sound/north-canada/lummi-island
[4] http://www.moon.com/destinations/washington/san-juan-islands
[5] http://www.peacearchequestrian.net