Regarding hotels, you could stay at the Hyatt in downtown(about 2 miles from PBI), which is very close to Clematis street which is a strip of restaurants and such, and is also near the intracoastal. You could be at the airport in 5 minutes.

…hyatt.com/hyatt/…index.jsp

http://www.westpalmbeach.com/clematis/

There is a Marriot that is very close to City Place, which is a shopping/eatery/bar complex. It is mostly outdoors, and a fun place to hang. It is about 1 mile from the airport.

marriott.com/hotels/…

http://www.cityplace.com/

On airport prperty is also a Hilton. From there, you could easily catch a cab to City Place, or Clematis Street downtown and catch a nice dinner and see a little of the sights.

…hilton.com/en/…index.html

Regarding Tri-rail, the main WPB station is right behind the Marriot that I mentioned above so is not far from PBI. As far as where it drops you in Miami, I am not certain. I think you get off at 1 stop, transfer to a people mover in Miami, tto the port possibly. I woudl research the cruise line as they may offer a shuttle of some sort for a fee as another option. There is also the Go Shuttle that could give you a ride from PBI. Google it. Sorry I do not know too much on Trirail down there. Maybe someone will chime in. Have fun.