Oh Natalie, surprising Grandma with a Walt Disney World trip for her special birthday is an amazing gift of love to her. She is a very lucky grandma to have loved ones help her celebrate. Some of my best memories of trips are when we took my parents there and watched three generations enjoy the magic together.
Disney does not offer gift certificates or vouchers but there are many other great ideas of how to surprise her. Get a Disney themed birthday card and include a
Disney Gift Card. You can choose from many different ones and if you know her favorite character I am sure you can even find one with that on the picture. The gift cards can have any amount on them so you can use it to represent the entire trip for her or just a souvenir amount to announce your plans, Either way I am sure she will be surprised completely.
If you are close by and plan to spend the birthday with her then I suggest you purchase a Disney themed cake and make everything Disney from wrapping paper to candles. Let the kids be involved if they are old enough and have them use their Disney imagination to make her day special. You can purchase a small Mickey Mouse doll to give her and after all of this she will really be wondering what is happening. In the end you can have everyone yell out the surprise....."What do we do when Grandma turns 70? We all go to Disney World!". If you can't be with her you can even do fun things via the phone or FaceTime.
She is one lucky grandma and I know she is going to be thrilled with your very generous gift.