Shakespeare in the Park is an option - everyone bring food to picnic, meet early before the show, and then those who want to watch King Lear can do so after the meet up. June 16- July 5th. Not sure if my hubby would go for that, though, as that is traditionally one of our few date nights together.
The Money Museum at the Federal Reserve is free and was thrown out as an option - Liberty Memorial is good, too, for a picnic spot and the museum is good (but not free).
Loose Park (I hear) is beautiful right now as the roses are blooming.
I think someone should pick a date and just go with it, even if it doesn't work for everyone. It'd be nice if we got something set up quarterly so that those who miss this one can go to the next one.