The train is the easy answer. Board Amtrak Cascades at Union Station, ride about 3.5 hours, and step off at King Street Station in Seattle. Several trains run each day, so you can pick the one that fits.
Driving? It's I-5 the whole way north. The address goes out to everyone enrolled once the venue is set, so you'll know where to park.
Flying? Alaska and Delta both fly PDX to SEA nonstop. The MAX Red Line runs from downtown to the terminal in about 40 minutes, so nobody has to drive you out there, and PDX's new timber-roofed terminal is worth showing up early for. The Seattle page covers the Link train from SEA into the city.
Lake Tahoe in August is the next closest training. Alaska flies PDX to Reno nonstop, then it's a drive up to the lake. Read the Lake Tahoe page. Salt Lake City in April is a nonstop flight too, on Alaska, Delta or Frontier.
September, and What to Pack
September is Portland's best month. Highs in the mid 70s, mostly dry, about seven rainy days, and October nearly doubles that. Seattle runs a touch cooler, highs around 70 and mostly dry, before the October rain. Pack layers and a light rain shell.
September is also swift season. The Vaux's swifts left the Chapman Elementary chimney after 2024, but thousands still pass through town at dusk on their way south. Bird Alliance of Oregon posts where they're roosting this year.