You don't have to dance. We just sat there and watched for about two hours. It was quite interesting/entertaining. Most milongas have liquor service, so you can get a bottle of wine/beer and just take in the surroundings.

We went to Salon Canning. It was nothing fancy, but the people were extremely nice and friendly. It was cool to see the tango culture from both sides -- theatrical and participation.

Jeez, just thinking about it all makes me want to go back ASAP.

(Bear in mind, Salon Canning is a little hard to find -- it isn't well-marked. That was the only milonga we visited. It doesn't pay to get there much before 11 p.m. or so.)