Hi Kathryn,
Happy anniversary to your parents. Disney is the perfect place to celebrate!
One of the easiest things you can do is get them a free "Anniversary" button at any guest services office in the parks. (These may also be available from your hotel front desk.) This will alert cast members and other guests that your parents are celebrating. There's nothing guaranteed, but sometimes they'll receive small perks.
A great way to honor your folks would be to get them a special gift from the
Disney Floral and Gift Basket Department. They have wonderful arrangements and gifts of food and souvenirs at all price points that could be placed in their room as a surprise.
Another fun idea is to surprise them with a cake at one of your table service meals. You can arrange this via the Cake Hotline at 407-827-2253. (I know, a cake hotline, how cool is that!)
For more tips on celebrating, you may want to head over to the independent website blog.touringplans.com and search there for an article called "Trip Planning 101: Celebrating at Walt Disney World."
I hope you all have a magical vacation!