Minnesota
Yes I have to agree with Nora. With the long winter it is always nice to get flowers that have lots of different colors. I don't think a favorite would matter much, the fact that you go out and buy them is what swoons us!! 
Madison
I also love to bring flowers. They are nice for a touring Lady at the beginning of her stay, as they will brighten up what is often an otherwise boring hotel room. Near the end of her stay, though, they will be enjoyed only briefly, then probably wind up going home with the chambermaid, since she can't readily take them on the plane with them.
Also, bear in mind that some Ladies' lives are such that they cannot easily explain returning home with flowers, at least not on a regular basis.
Finally, some Ladies may dislike certain aromas, or even be allergic to them.
As always, best to pay attention to your Lady and discover what SHE likes. Being thoughtful is wonderful, but it actually requires thought
Certain "gifts" can only be given by certificate. I gave a certificate good for a half-day at a local spa to a special Lady on a special occasion, and she seemed to appreciate it. I also arranged a Dinner Cruise for a Lady and her children on a beach vacation they were taking, and simply told her where to pick up the tickets.
I once had the hotel massuese visit a Lady's hotel room at a time I knew she would not be otherwise occupied (although I did warn them to call her first, just in case!). The massage preceeded my arrival by about an hour, and I will say that it proved to be an excellent investment, as well as a nice thing to do for a very nice person
My guess is that dollar-denominated gift certificates are a less personal than a certificate for a specific gift, but still more personal than money. I have always felt that way as a recipient, although I'm sure I haven't had even a small fraction of the experience receiving gifts that some of these wonderful Ladies have. Actually, the Ladies give me a very special "gift" every time I see them - I'm just trying to return the favor 