Plaza Castelli is the living room and beating heart of Belgrano R. Like Palermo, Belgrano is a massive neighborhood with different pockets with unique personalities. Belgrano C is commercial and busy. But this is Belgrano R (these names come from their train stations).
Belgrano R is peaceful, with large sidewalks and tree canopies, large homes and luxury apartment blocks. You're likely to see families and moms pushing strollers versus busy people in suits rushing to work.
I say this because I was once this mother pushing her kids in the stroller to Plaza Castelli's playground. I have spent hours of my life here, so you could say I'm biased when I say it's lovely.
The playground in the center is basic but popular. Avoid it in summer as it has zero shade (ironic as the rest of the plaza is completely shade covered).
Lining the square are beautiful European style apartment buildings and cafes. There is an English language bookstore on the corner - KEL - in case you need to pick up a new read.
TIP: Walk a few blocks to nearby Avenida Melian, the most beautiful avenue in the city. Completely covered in huge tipa trees with cobblestone and lined with single family homes it is heralded by many (not just me, I swear) as one of the most beautiful avenues in the city. (See two photos above for proof).