Mt Rainier and Mt St Helens are both totally spectacular and end of June or early July might be perfect time for wildflowers, depends how snowpack fares this spring.

The air and space museum is cool. I haven't been but people really like the San Juan islands.

Right in Seattle the Hiram Chittendon locks (Ballard locks) are actually interesting.

As others have said, Space Needle of course. Last time I was there I went up in the evening before sunset and so got to see everything in daylight and then the transition to night with the lights, was really cool. Super busy though, might be a bit problematic with a 2 year old. If you go, check out reserving ahead of time.

If you wanted to come towards Portland check out the Gorge. Could make a loop and go to Timberline Lodge as well. Nice view up there and an overall scenic drive.

Oregon coast is overall better than the Washington coast if you wanted to wind up out there.