"Lively, but a bit dark"
Park ave south is the section of Park ave that stretches towards Union Square. Although The Square itself is usually quite lively, Park ave itself is quite dark. Not many stores there stay open at night, although there are some great restaurants to check out, such as flute, and many sushi bars. It's a pretty decent area, but I would recommend going further east for some more going out options, or just basking in the intensity that is Union Sq. If the weather is nice, you can always spot a great deal of people just hanging out, dancing, listening to music on the steps of the square. It's a great spot to just chill out, even after dark, because the people there are friendly and, most of the time, they enjoy meeting new people. If all else fails, you can pop into Babies R Us or Barnes and Noble for some random shopping!