What to do in Alexandria at the meet up? I'll give it a shot...
How about a walking tour of Old Towne with the Yorkies invited? A visit to the closest dog park for little dogs is always fun. A tour of the monuments in DC with our Yorkies (they will not be allowed in some, but with lots of us going, and not all the Yorkies attending
we will have plenty of doggy sitters.
Renting bikes and riding to Mt Vernon on the George Washington Parkway trail is a lot of fun from Old Towne. Could bring some of the little guys too with the right equipment (they would not be allowed in Mt Vernon of course). Then there are the humans-only traditional DC activities, such as touring the Smithsonian or other mall attractions. Kennedy Center, River cruises, side trip to Smithsonian Air and Space Annex at Dulles, attend a Nats baseball game; there are many options. A trip to Great Falls could also be a lot of fun and makes for a great hike (lots of dogs too).
What needs to be done IMO is list potential activities and canvas interest from the group of likely attendees.
Shawn