I also like the poem idea. Another option is to include a small card (business card sized or smaller) mentioning where they are registered or that Disney Dollars/Disney Gift Card are wonderful options as they are HMing at WDW. Although doing this does not fit proper wedding ettiquete, but it does provide guests with ideas.
I made an attempt at a poem for your invite mentioning Disney $. But beware it is very corny as it was done rather quickly. It's just to give you an idea that's all.
Jack and Sally are about to embark on a journey that is like no other,
They are to sail the seas, and see the World with eachother.
A perfect gift would be that of Disney Dollars to use in this special land,
As a way to send them off, to all that is planned.
But this is no everyday money,
But money to be spent in the company of a honey.
A honey of a Pooh Bear, and two mice with big ears,
After a week of memories, that will be remembered for years.