Bikeable beaches...

Yes, all of them accessible. As is Riis Park (sp?). Depends what kind of audience you're looking for I guess...

I always took the longer route to Coney Island - along the bike path that runs along the Belt Parkway. The only "iffy" part of the ride may be along Bay Parkway to Stillwell. You can either take Cropsey Avenue all the way down (a little too much traffic) or ride along the service road (a little desolate).

Here's my five cents:
1. All bikeable. Depends on how far you want to ride. Manhattan Beach straight down Bedford is probably the closest.
2.Free NYC bikemap is available at bikeshops and it's pretty useful. Lists all bike lanes, greenways and has suggested routes.
3. Avoid Flatbush Ave to Riis or Rockaways unless you like riding in traffic. Cars speed like mad from Kings Plaza to the Belt and north of that you will be contending with truck deliveries and buses, etc. It is the most direct route though.
4. Bedford bike lane is very chill, from Foster Ave down to the beach, particularly in Midwood on Saturdays. North of that you will hit more traffic and potholes. You could avoid that section by detouring through Ocean Ave and then snaking over to Bedford around Foster or Glenwood etc.
5. Have fun.

One last tip if you ride to Riis or Rockaways or Canarsie Pier on the Greenway from Sheepshead Bay through Plumb Beach. If you cross at the light on Flatbush which takes you over to the path on the Floyd Bennett side of the road, please wait and make sure that cars are stopped or stopping for the light before moving into the street. I can't tell you how many cars I have seen blow that light. They don't expect it to be red (it's an intersection only for greenway users, and drivers are often going fast on this stretch.) The light only turns red for vehicle traffic when you press the crosswalk button.

I love this Greenway and ride, but I always wait just a hair longer than normal at this intersection now.