North Clackamas Aquatic Park is the only indoor water park in the Portland metro area. There's a heated wave pool with 4-foot waves, three water slides, a 25-yard lap pool, a diving well, and a hot tub; during their public "Big Surf" sessions, you have access to all of it.
On Fridays in the spring, they're only open from 7:00pm to 9:00pm, but it's cheap: just $5.00 for two hours of splishing and splashing. (Regular price on weekend days is $7 for residents of northern Clackamas County and $10 for non-residents.)
Bring a swimsuit (not gym shorts or cutoffs) and a towel. Lockers are available for a $1 rental fee, as are inner tubes. Outside food and beverages are not allowed. Drowning is strongly discouraged.
I've scheduled the meetup to begin at 6:30 so we can rendezvous in the lobby, get organized, get changed, and make the most of our Big Surf session!
By the way, if you'd rather stay dry, the indoor rock climbing wall is also available during Big Surf sessions; prices may vary.
Carpool! If you need a ride or can offer one, please post in the comments below or in your RSVP. Public transportation is available too; the Aquatic Park is 1 mile west of the last stop on the MAX Green Line.