My in-laws did this for our family too

. It was a fabulous trip. What I did was make them each something special to wear on our trip. I made my FIL a polo shirt with an iron on Mickey

patch and his initials. I made a matching one for my son (the only grandson) too, so they could wear them together. Then when my MIL mentioned she had been trying to find a belt that was Disney themed to wear while we were there, I made her a ribbon belt in red and white with black Mickeys

stamped on it. It was a huge hit with her and she wore it every day. You could also get them something while there to present to them when you get back.
You could also call your resort, if you are staying on site, and ask them to have something waiting for them in their room when they arrive. Maybe some personalized mouse ears

.
You could take them to the Hoop-de-doo. My IL's loved that.

. I think that a night at V&A is a great idea or tickets to Cirque de Soleil.
I think whatever you do the fact that you thought to give them something in appreciation is all that matters.
