The best day and time to visit the Guggenheim is Saturday night from 5:45PM to 7:45PM. The museum is open late (though not late enough in my opinion) and the admission fee is reduced from $22 for adults to the very reasonable "Pay What You Wish." If you're on a budget or traveling or otherwise aren't swimming in $22 bills, it's a great time to get uptown and experience a wonderful museum for a nickle or a dime or a dollar or whatever you think is appropriate. There's usually a line, but don't worry it moves quickly.
On a recent evening, I went to see the retrospective of the Dutch photographer Rineke Dijkstra. It was exceptionally well-curated and is a highly recommended photography show.