Assuming money is not a factor: The private dinner at Grand Floridian is a great idea, it includes a custom menu, a butler, and there is only 1 reservation a night. It is generally around $500 for a couple and I can get you more info if you are interested. Another great option would be the two night in room celebration is amazing and I am doing it for my Fiancée the night before and the night of our wedding in WDW. Here is a link for it:
http://disneyworld.disneyfloralandgifts.com/category/in-room+celebrations.do?nType=1, it includes flowers on the first night, with a nice card and some chocolates I believe, then the second night it includes a silver bracelet, champagne, electric candles, and rose petals all over... I want to say it was $400, it includes more but I honestly can't remember what lol...
Ok so being more budget appropriate for most people you could have flowers delivered to her throughout a day, for example each hour have a flower delivered to her at a certain point in a park with a special note
I can't speak for in park but I again am having this done for my Fiancée the day of our wedding, I am having flowers/items delivered to her every hour with a note... This was kinda hard to setup, our wedding coordinator ended up having to get a Disney Floral Manager to call me because they were trying to just deliver it all to the resort at once and have the resort deliver the flowers 1 by 1 from the vase they assembled... the cost on this is for 5 deliveries to a hotel room was about $220 including a Mickey dozen roses, a rose wand, and one of those mickey balloons which have a mickey head balloon inside a clear balloon... I am not sure of the logistics of doing this outside of a single hotel room or even if it is possible but it's an idea
Disney is very romantic even without spending money though if you really want to make it special you may have to spend some
These are just a few ideas from my experience (either things I have done, am doing, or have extensively looked into doing) Hope this helps, let me know if you have questions about any of them and good luck!!!