Russ, I think this is a great idea and I'm interested in meeting up "IRL" as the kids say. But can you edit your question? I totally didn't get that you were asking for input on a QL meetup based on the question alone - something like "Qland Birthday Meet-up April 1: Who's in?" (Is it too late to edit? Maybe the Gnome could help with that?)
While I love the idea of QLanders mobbing a ferry ride, I think it's too much of a time & money commitment for most people, even if we immediately came back to Seattle. I would suggest the West Seattle Water Taxi as an alternative cruise, but it doesn't run on the weekends till later in April.
Downtown or very close to downtown is best for me, unless people are interested in meeting in West Seattle, because I'll be busing. Seattle Center works for me, but I would rather meet somewhere with an indoor option - something like a restaurant or meeting room with a patio, if people want to have an outdoor option. I just have my doubts that the weather is going to be nice enough to meet by the fountain, and I can't think of anything indoors close by except the Center House, which is not a place I would want to meet up unless it's been remodeled.
What about a cafe or restaurant? Someplace where people wouldn't be obligated to buy something but could just hang out, and someplace Marty's kids would be welcome. I'm totally busy with finals right now; I would offer to check around for options but I don't have time right now. Especially since I've been procrastinating on Qland so much. :)