You probably don't have to get your nieces and nephews excited about going on a cruise. They're probably already stoked. But I might pick out a cruise ship playset or two to keep in my playroom. After we get home from the trip, they can continue the fun and enrich their memories by acting out our adventures with a fun cruise ship themed playset toy. There don't appear to be any cruise ships geared towards boys - these are mostly dollhouses.
So far I'm the most tempted by the MegaBloks Hello Kitty cruise ship. You do have to put it together, but it's far less pieces than it would be for Lego or Playmobil. The cruise ship opens up like a dollhouse, and features three levels. It comes with a Hello Kitty figurine, of course, and a few little friends for her. There's a cute dining area on the front of the boat, bedrooms for the crew, and a little cafe with parasol on the back of the ship. Darling!
For a younger niece (say 3 or 4 years old) here's a simple Hello Kitty cruise ship that you don't have to put together. It's not very exciting, but maybe it would be for a three year old, especially if you played with her enthusiastically.
I would probably only get the Polly Pocket cruise ship if I had a niece who was already obsessed with Polly Pocket. Check out the cool water slide, and the funky fuchsia "ocean waves" wallpaper.
The Lego Friends Dolphin Cruiser is really cool. However, at 612 pieces, I would only get this if I had a niece old enough and with enough patience to help me put the dang thing together. I love that it's not too "pinky stinky" like some of the Lego Friends toys can be. The cruise ship playset comes with a couple cute dolphins, a Sea-Doo or Jetski type water vehicle, tropical fruity drinks and a couple of sun loungers. One of the drawbacks is it seems once this is put together, it would be hard to put the little figures down inside the interior of the ship. This might not work well as a dollhouse-style playset.
If your niece likes Barbie dolls, here's the Barbie Sisters cruise ship. It doesn't look like much on the outside, but opens to reveal two sun loungers, two water slides, a pool, a swing chair, two bunk beds, a little dining area, and a precarious perch for the Barbie captain to steer the ship from.
For little babies (age 1-2 years old) here's a cute Disney plush cruise ship. It comes with Mickey and Minnie plush dolls you can store inside it.