If you're visiting Portland in July or August, consider driving to the nearby suburb of Lake Oswego to swim at their swim park. With a lap section, wading section, and even water cannons, it has a fit for everyone.
It's a community swimming pool, but we didn't have any trouble visiting as out-of-town guests. It was a great way for the kids to spend some energy after leaving Portland (on our way back home to Southern Oregon), or when we're just driving through on our way further north.
You can bring swim toys such as rafts and boogie boards, too. My kids wanted their swim goggles, so we'll remember those next time!
Afterward, you may want to stop by the Hanna Andersson outlet for some great deals on children's clothing! (7 Monroe Parkway)