Just about any US cruise can be combined with a stay just change the flight dates, internal hop also means Vegas is often not that much more expensive than the return.
For New york , the canada cruise runs are a good choice.
Don't see the point of going south from NY might as well fly to florida and just change the flights via NY.
A transatlantic to/from NY with a stay is another option that is much cheaper,
We did 9 nights NCL Gem into Boston with flights min-suite £500.
Just use vacationstogo.com and put in Newyork as a departure point or a port of call and see what comes up.
Another option is pick a cruse and try to sort out flight around it open jaw flights are often ok so you could do a panama cruise, fly to Miami cruise to LA, fly to vegas then onto NY then back to the UK.
Basicly you can do what you want on any cruise line.