Palermo Woods. Entrance is free, and if you ride your bike there you get a free water. The ad doesn't make it sound epically exciting, but the MTV sponsorship should at least promise some good music and an interesting crowd. If you don't have anything better to do, check it out, listen to music and confirm that in fact, some people in Buenos Aires do care about the environment.
The second event is downright irresistible, a farmer's market/food feria in San Telmo on Saturday and Sunday from 10 am to 6 pm, at 400 Avenida Caseros. Calling all foodies, this event sounds fenómeno. According to the poster, "during two days, 40 producers of healthy foods present more than 400 products to taste and buy at promotional prices." We'll see about the promotional prices... Anyway, the event seems to span the entire food world including beer and wine, produce, spreads, pastas, breaks, snack, kitchen equipment and more. FYI, the rain date is May 5 and 6, and there's rain on the forecast.