I second flatiron lounge. I would NOT recommend tenjune for a pre-dinner drink, unless you enjoy clubbing before dinner. Flute is also a good idea for a birthday- as the name suggests its a champagne lounge.
i just figured since Tenjune is a lounge/scene it would work for the OP's request. is Flute still considered to be "trendy" after all these years? (regardless, it *is* a nice idea for a birthday cocktail).
Also, if its nice out, 230 Fifth has a great view, a big "after work" crowd, and is just up fifth avenue. Or try Olives at the W Union Square which may seem obvious but does still have a decent ambiance.