A one day subway pass is $7. A weekly pass is $21. One-way fare is $2 so if you're making two round trips a day it is definitely worth it. The subway is the best way to get around...I always take it when I'm there.
As for places to eat...Confetti Pasta is right near that church! It's on 38th Street near Madison. It's a good place for lunch. You pick a pasta, a sauce, and whatever you want in it (tons of choices!!!) and you get that plus salad and bread for something like $8. Big portions and it's good. There's a lot of eateries in Grand Central Station itself. You can also take the shuttle over to Times Square and find zillions of other places to eat.
Have fun! I love NYC.
