Hi there!
Not sure how big you are looking to celebrate your 25 years but you could do a Disney Vow Renewal. If you are interested I can share some details with you.
If you aren't looking to go that big there a number of romantic things to do. For about $35 you can take a horse drawn carriage ride at either Disney's Port Orleans Riverside Resort or Disney's Fort Wilderness Resort. You could also purchase a bottle of champagne to have on the ride.
There are also a number of romantic places to eat around EPCOT's World Showcase. You could have a fancy dinner and top it off with Illuminations Reflections of Earth.
I'm not sure how much they cost but you could rent a boat and have a Wishes Cruise on seven seas lagoon. I am sure some others on this board have done that and could help with further details.
Those are just a few things off the top of my head. Feel free to email me or pm me anytime with questions or anything!!
Have A Magical Day!