From what you describe, maybe a large family gathering would be in order. Gather the kids and grandkids and anyone else...eat, talk, laugh, and play all day. Throw in a vow renewal.
Maybe you and the kids/grandkids can put a family scrapbook together and have everyone write a note to include in it....expressing their memories and thoughts regarding y'all's 25 years together, and wishes for many more. Give it to her at the gathering.
I like this ^.
And the theme for the day should be Mrs. GED and how who she is and what she's done has helped make all that you share on that special day, and every day possible.
Think about the things you said on this forum about her, then tell her, ( maybe write it like someone else suggested ) how it made you feel, ( lucky, proud, blessed, etc.). Tell her how it feels to know she's there.
As for a gift look for something that will change something in her life and make it better on a regular basis, is there a sun shade on that lawn tractor?, a new mattress can do wonders, does she sleep good? Anything that makes her housework/cleaning easier or quicker is a possibility.
Regardless of any gift you give her if it's in your heart tell her that any gifts pale in comparison to what's she's given you for the last 25 years.