This is pretty specific but...
For any single guy who is on the verge of engagement, 47th Street in NCY is an AWESOME place to buy an engagement ring. Plus, if you have them mail it to an out-of-state address, it is treated as a mail-order purchase... no sales tax added to the price!
(I know, I know... your home state most likely requires you to voluntarily pay use-tax on the ring, and anything else which you purchase from out of state.)