Well I'm from NYC and the best thing to do in Summertime is to walk around the city. You can do all the shopping you want from all the different areas in the city. It may not be quiet as you would like, but it is a cool experience. I also heard from friends of mine that the poconos is very nice place to go for fun in the sun.
Good luck.
P.S. The summer is great for some free events from time to time. Like free concerts in Central Park or in Washington Park in Greenwich Village. It usually pays to look around for great deals.