my top pics would be- (not in any particular order)
Un Altra Volta- anybody who doesnt like ice cream, in elegant surroundings, needs counseling.
El Ateneo- the coolest bookstore in the world. Takes your breath away the first time, you cant stop looking up.
A walk on Corrientes on a Saturday night- bookstores, record stores, restaurants, theaters, and best of all- lots and lots of Porteno's, being Porteno's.
Museo de Arte Decorativo- the grandeur of Argentina at its height. Have tea or coffee and a snack at the cafe in the garden.
Museo Xul Solar- crazy architecture, the hidden argentine soul, wild art.
A Bondiola sandwich from the pushcart on Dorego, in the Bosque Palermo. A beautiful setting, a grand park, great cheap food.
One of my favorite things to do, though, is just take the collectivo to a new barrio I havent been to before, and wander around. Eat, of course. I am working my way thru this book-
http://www.littlebookroom.com/authenticbuenosaires.html
there is usually a destination in most any neighborhood.