Of course the referrals are a given - that is their bread and butter after all. However, a nice little thank you gift brighten anyone's day. The flowers, restaurant certificate etc. will surely be appreciated. When we made the final payment on our last cruise, I found a very nice coffee mug at the office supply store for $10. It was very nice with a world map design and some sort of travel saying around the rim. I thought it would look nice on her desk for coffee or even pencils.
Maybe you could ask her co-workers what she drinks, and pick up a bottle of duty free alcohol from the ship or one of the ports. You can also purchase a "cruise" style picture frame on the ship for her desk.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days