Tubing is also popular in the Clackamas river. There's a sporting goods store called Next Adventure that might be a good resource for tubing.
A popular side trip to the coast from the Beaverton/PDX area is to go out highway 26 to Seaside/Cannon Beach, then North on 101 to Astoria, then back to the city along highway 30 (which runs along the Columbia River. Or just go out and back on 26. Cannon Beach is the fancy beach town, Seaside is the family friendly "touristy" beach town, Astoria is the old river/hipster town. In Astoria you can climb to the top of the Astoria column and fly cheap balsa wood airplanes off the top that you purchase in the gift shop, kids love this. (If you notice the wind patterns, you can find all the used airplanes in the woods nearby!) The Maritime Museum is fabulous but might be a little boring to young kids? There are usually sea lions lounging at Pier 39 that are popular with visitors (most locals hate the sea lions!)
A really great state park is south of CB-Oswald West, the beach there is known as Short Sands. It's a short hike from the parking area to the beach. The water is too cold for swimming without a wetsuit, but it's a gorgeous spot. Or you can stop at the Hug Point wayside south of CB but north of Oswald (parking lot next to the beach) for a similar experience with less hike to the beach. Fun at the coast can be really weather dependent. It can be 85* in pdx and a drizzly 65* at the coast. Seaside gets a bad rap for being too touristy, but there's lots of fun stuff to do there, even just walking along the prom (the boardwalk, essentially), to the candy shops and arcade. Lots of restaurants, too. Doogers is good for seafood (fried razor clams!) Cannon Beach has fun tidepools to explore at low tide near Haystack Rock.
OMSI is great, as was mentioned above. The zoo is popular, maybe too popular. I always joke a summer weekend trip to the zoo is really powerful birth control.
I don't know the skate scene much, but I know there are lots of parks there. All the towns mentioned above have them, too.