to get there: from Penn Station take the Long Island Rail Road from Babylon to Freeport (a round trip ticket is 15 dollars). from Freeport get on the MTA bus headed to Jones Beach. the bus is the cost of a subway ride and if you have your unlimited metro-card (which i'm sure you've figured out by now that you should) then it's no added cost. all together it takes about 1. 5 hours to get there from when you leave Penn Station (shorter if you don't have wait to long for the bus).
to do there: soak up the rays, bob in the waves, and get a big ol' scoop of some Friendly's ice cream (mint chocolate chip for me, please).
the beach is really quite beautiful and while (like Coney Island) it feels a bit past it's prime and makes you long for the simplicity of years gone by, it is nonetheless a welcome-adventure.
an easy day trip, i might not suggest this for visitors to the city, but rather for the hardened resident looking for an escape.
plus, it's a perfect excuse to pull out that wide-brimmed hat you've had tucked in the back of your closet for two years.
and don't forget your spf (70 for me please). uva/uvb. i may not be the tannest of them all. but i'll laugh when at 40 i have barely a wrinkle on my face. and more importantly, skin cancer can be prevented...so, why wouldn't you arm yourself against it?