Swifts
Every September, the largest gathering of Vaux's Swifts in the world happens right here in Portland, at Chapman School in NW. A dozen years ago there were 35,000 birds; more recently the peak has been 15,000. That's still an astonishing number, particularly when they're all flying right above you.
If you've never been: go!
Additionally Wallace Park is adjacent to the school, has a big playground and open field. There is also a dog run and a covered picnic area. You could make a whole afternoon out of it, if you wanted!