When we received a press release a few months ago about the expanded 15,000-foot playground re-opening in Union Square Park, we thought . . . okay, another new playground. Sounds good. But when we checked it out on Sunday (with some junior “assistants”), we found a truly special oasis of child activity and merriment. — The people who designed this space really have learned from the last 10 years of advances in playground fun and functionality, and just about every feature is the best of its kind: the obstacle-accessorized climbing vines, the slides bursting out of rock quarries, angled twirly hang-on-if-you-can spinners, and the crown jewel, the stainless steel mountain orb (the first in the city). The playground is divided into a space for smaller children up to about age 4 (with the requisite sandbox), and big kid area, which has enough great stuff to challenge children from ages 5 to 12. If you don’t live in the area, this playground is worth treating as a destination spot, especially since you can combine your trip with a picnic in the park, shopping at the Union Square Greenmarket, and visits to Barnes & Noble, Books of Wonder, Max Brenner, and the other spots in the area. PS: Let your child meander through the mini flower garden by the statue of Ghandi at the other end of the park—and try to explain to them the importance of Ghandi! PPS: The area near the playground will have a water park with a big fountain and misters as the weather gets hotter.