Skip Grimaldi's unless you like tourist traps. If you really want to bad, go over to the Grimaldi's in Hoboken, no line, and walk along the Hudson River Park for wonderful views of Manhattan, esp. at sunset.
BUT, how can you possibly have nothing to do? If that's really the case, try some food carts, try a high-end lunch (Per se, Le bernadin, Bouley, etc.). Fun Mexican = Rosa Mexicano. http://www.rosamexicano.com/