Hi, Jacqueline! I am planning a similar journey myself this year, bringing my stepfather with us on an upcoming trip to create the memories with my children that my mother would have wanted to had she not gotten so ill and passed away unexpectedly. They are memories you will cherish with your mom, and I know how important it is for the trip to be special.
Think of the things she was hoping to do with your dad. Was there a special dining experience she wanted to enjoy? Were they hoping to see the Wishes fireworks at the Magic Kingdom together after a fun day of enjoying the attractions? Did she want to do some behind the scenes tours? Give some thought to the things she was looking forward to doing.
Definitely consider purchasing
Disney Memory Maker, which will give you access to photographs taken throughout your vacation, including some special pixie dusted ones. This will give you and your mom a lifetime of photographic memories from this vacation.
You can make a wish or say a prayer and toss a coin into the Wishing Well behind Cinderella Castle. The coins collected are donated to children's charities, but it can be a sweet way to think of your dad during your trip. Purchase some seeds or a seedling at The Land pavilion at EPCOT and plant them when you get home in honor of your dad.
Have t-shirts designed with your Dad's photo on them, and wear them into the parks. He will surely be with you in spirit.
Know that I'll be thinking of your family as you make this bittersweet trip. I hope you start new traditions and make happy memories!