Some of the obvious things would be the various theatre marquees, the TKTS board, the statue of George M. Cohan. You could also go for other significant buildings (like the entrance to the Equity building) or architectural details (like the statues of Ethel Barrymore, Marilyn Miller, Mary Pickford, and Rosa Ponselle that were restored when the I. Miller building was renovated and Express moved into the ground floors), or certain types of people (like the flyer girls for Chicago in their character shoes and red tights).
Can people use their phones? Are you going to use clues or do they just have to find something to take a picture with? For instance, you could ask them to take a picture in front of/inside the church that used to be a Broadway theater. Or you could just ask someone to find the M&M store.
specific characters (Elmo, Minions, a living statue, etc)
someone wearing show swag
one point per marquee
one point per show flyer
a historical marker
something from Jack's
graffiti
throw in some generic stuff like a dog, a pigeon, red shoes, a bicycle, etc.
Or you could get clever with it and make every item a reference to a Broadway show. Can you snap a pic of a guy with a doll, kinky boots, a (weird and unusual) plant, a dinglehopper, etc.