Hi Charlie, I'm from Seattle and I just saw this, so I haven't thought too much, but right off the top of my head, I think one of the local farmers markets could be a great venue for your band.
http://www.seattlefarmersmarkets.org/
There are a lot of cities in the Puget Sound(Seattle) area within ~10km or so of Seattle that have their own farmers markets too. Vashon & Bainbridge Islands and short ferry rides from Seattle and they also have nice farmers markets. There's also the famous Pike Place Market downtown, but I'm pretty sure there's a permit system in place to busk/play. Maybe you could try to find a place to play in Olympia on your drive South to Portland. I'm sure there's a good place for a permaculture ukelele show in Olympia.
Good luck!