Trains or not Portland is worth the trip! Personally, I wouldn't take Amtrak down there - the line is plagued with delays and takes considerably longer than driving (almost 6 hours last time I made the trip!) A bonus is that there's no sales tax in Oregon and lots of great places to shop for all sorts of handmade items. Also tons of amazing restaurants (ditto for Seattle). We usually go down there at least once a year.
Train related trips:
Head across the lake to Eastside Trains. Simply the best store in the PNW and they mostly deal in O gauge. Another good day trip is to head to Leavenworth. The drive takes you over the Cascade mountains and Leavenworth is to Washington what Solvang is to California but with a German theme. On the way to Leavenworth be sure to check out the Great Northern & Cascade Railway (http://www.gncrailway.org/). They operate a 1/8 scale layout you can ride on and it's completely free though please make a donation. They just started running one loop last summer and they have ambitious expansion plans. Skykomish is pretty much a one street town and this railroad park is their attempt to reinvigorate the place. The Mt. Rainier Scenic Railroad is also a nice day trip to the south. Taking the Sounder commuter train 25 miles north to Everett is a cheap and scenic trip. The train runs through the BNSF railyard just north of downtown Seattle, over the Ballard shipping locks (home to most of the boats you see on Deadliest Catch) and pretty much hugs the waterline all the way up to Everett. It's arguably the most scenic commuter railroad line in the nation. Everett is a naval town and home to the Nimitz carrier group.
Non-train related:
Another good day trip is to take the car ferry out to one of the islands. Bainbridge and Bremerton are both nice and are the two closest. If you bring your passports you can take the high speed ferry to Victoria on Vancouver Island (not to be confused with Vancouver the city which is on the mainland!) The recently opened Museum of History and Industry (MOHAI) in Seattle is worth a visit as is the Museum of Flight just south of Seattle at Boeing Field. If you like beer be sure to check out some of the local breweries in Ballard or Georgetown. One great thing about Seattle is that some of our best family friendly restaurants are at the breweries.
July is probably the best time of year to visit Seattle. So far we've had an unusually warm and sunny spring so we should be avoiding our dreaded "Junuary" weather! Also be prepared to deal with some of the slowest drivers you'll ever encounter. If you visit Eastside Trains (or anything else on the east side) absolutely DO NOT SPEED! Everyone I know who works on the east side has gotten speeding tickets for going 3-4 MPH above the limit. Unlike California, this is completely legal in WA.