There's a Boxer play day the second Sunday of each month at North Clackamas Park near Milwaukie. We haven't made it over there yet but keep meaning to.
We take our 2 to a doggy daycare place in Gresham once a week to socialize and wear them out. I know there are a couple of places like that closer to downtown Portland.
Oh, also I read about a new boxer-themed coffee shop on your side of town called the Iron Mutt, and I want to get over there to check it out! They might have connections for you with other Boxer people in the area.