Ella, you can send gifts to their staterooms, go to Princess Web Site you will find all the info there, there is also another company that specializes in gifts for cruisers, I can't think of the name, but do a google or yahoo search and you will find it. Its like a Welcome Wagon type thing.
The easiest thing to do is to go to their travel agent and let her make the arrangements.
When you buy these packages yourself you will need their name, ship name, sailing date and cabin number, not sure if you need their booking number - if you want it to be a surprise your best bet would be the travel agent.