Thanks to all for the recs. We just got back. The trip was great! Not a drop of rain the whole time.
So, in case someone later on finds this thread via a forum search, some things I found out.
-For open water swimming, the Madison Park Beach on Lake Washington worked out great. I went their twice, the first time with my wetsuit and swam a mile or so along the coast; the second time I just stayed within the roped-off area, but even that is around 100 yards long. Relatively early in the morning, but it wasn’t fully empty, which was somewhat reassuring. I just left my stuff on a bench by the shore and it was still there when I got back.
-Urban running is cool here, esp. if you like it HILLY.
-Green Lake is like a fitness neighborhood. Running stores, bike stores, crowsds of runners.
-Elliot Bay bikes is indeed super cool. It’s not big, but it’s got a whole bike museum aspect - a lot of classic bikes hanging from the ceiling. One even has that campy shifting system where you have to activate the quick release while riding.
(didn’t make it to Il Vecchio when it was open; looked in the window while it was closed, some neat bikes but small).
-Elliot Bay books is also excellent - one of the best presented book collections I’ve seen, with a lot of notes from employees.
-Seattle Running Company had one of the most impressive collections of trail running shoes I’ve seen.
-I didn’t quite get the Theo’s Chocolate thing. Nice store, but not on its own worth a trip, IMO, unless you call ahead and get the tour. But the neighborhood (Fremont) was really cool, so I’m glad we did go. But I guess I only like chocolate, don’t love chocolate like some people do.
-For anyone planning on hiking/camping in the area this year, a lot of stuff is still snowy, and it’s not clear that trails will be intact when the snow melts. Some roads that are supposed to be open in summer aren’t.
Other notes on Seattle:
-Everyone stops at the don’t walk sign! People don’t jaywalk. This was disconcertingly disconcerting. I didn’t think it would throw me as much as it did. Note to self: I’ve got too many east coast tendencies. The flip side is well worth it: cars are really good at stopping for you at crosswalks.
-Very impressive number of people getting around on their bikes.
-It’s a really good place to eat. Not as good a place to drink beer as I’d thought it would be, but a really good place to eat.
-Some of the food here is really, really spicy.
-Unlike Portland, where it is frequently rainy, the weather in Seattle is always sunny and in the 70s, although it does get cold at night. 
-Finally, the shocker of shockers: I thought that the espresso was better in rural Washington than in Seattle itself. Tried 6-8 different places, the best shot I had was in Enumclaw (and cost 92 cents!)