Jon suggested the 17th and that is doable for me, I think. My kids have a chess tournament, but I think I can get my hubby to cover that. I love the A&S because the lighting and subjects are fantastic. Hard to take a bad picture. I haven't tried my 5D there so I am very interested. I hope to get amazing shots with the 5D 17-40 combo. May not even need it. The 24-70 is really great there, but I worry about the DoF. Anyway, there is a cafe there for snacking and parking is the only fee.
The other choices sound interesting. The blossoms are my favorite and midweek, like we've been doing, is really the best. I recommend planning it so we can walk around and get several different angles. I'm thinking about bringing just the necessary gear for that one. It was almost perfect the last time.
I'm also game for Mount Vernon as it has been completely renovated and wonderful.