right there in the financial center, literally around the corner and a two minute walk away, there are a few restaurants that overlook the water. if the weather is nice, they usually have chairs and tables outside and it's kind of a nice atmosphere. I can't really think of the names right now, but there is one mexican restaurant, I believe, and a few pub-like places.
if you like indian food there is a baluchi's at 275 greenwich st.
another good place is Petite Abeille at 134 Broadway, inexpensive and tasty.
a bit farther but still walking distance is Les Halles on John St, across Broadway.
also Duane Park Cafe at 157 Duane St is pretty good.
You aren't far from the south st seaport, just a short walk, and down there you'll find more chain-type restaurants but it can be fun to be down there on a nice day.