It's almost never a hit or miss with parks in Argentina, there's a very delicate balance between…read morehow they look at different times of the day and how safe they make you feel. Whenever I review, I tend to do it imagining someone walking alone at night (as I tend to do) and therefore it's difficult to give a higher score to some public places.
Plaza San Martín is pretty big, with small slopes, tons of greenery, monuments, wooden and cement benches, and plenty of areas to relax, have fun or just walk by. It's surrounded by lovely buildings, and the higher area overlooking Retiro and the Torre de los Ingleses is quite charming. It is frequented by all kinds of people, and you can find tourists glancing at maps while a homeless man disposes of boxed wine at a nearby bin while teenagers skateboard nearby.
There's free wifi like in most public parks, but I've never been able to get a decent connection. Having said that, if you walk towards Maipú and Av. Libertador you can munch off of Starbucks', but you didn't hear that from me.