My bf and I are 25 and 26. There's so much to do there but we love:
Dinosaur at AK is my all time favorite, followed by Splash Mtn, Rock n Rollercoaster, Tower of Terror and Big thunder Mountain for big rides. For slower paced rides we love the Haunted Mansion, Pirates of the Caribbean and Peter Pan's Flight
JellyRolls @ the Boardwalk - dueling pianos, they play your requests and everyone gets into it, it's awesome
PI - favorites here for a relaxing night are Comedy Warehouse and Adventurer's Club. For a night of hard partying definitely Mannequin's, 8 Trax and the Beach Club!!! Mannequin's has a revolving dancefloor and Beach Club's live bands are usually great. And never underestimate dancing in the streets!
OHANA!! - best best best food on property for dinner. All you can eat skewers of grilled steak, beef, pork, shrimp, chicken wings, wontons, rice, etc. it's so good & if you set up your dinner 40 minutes before the MK fireworks they will dim the lights in the restaurant and play the music too, it's excellent
Mini Golf - I've heard the term PGA putt putt used for Fantasia Gardens, one of the courses is very difficult the other is fun. And Winter/Summerland is my favorite in front of Blizzard Beach.
Resorts - don't forget to check out the different resorts. My favorite is Boardwalk but all of them have different themes. We usually set aside 1/2 day just to check out what's going on at them. Wilderness Lodge and Campground also has a campfire for roasting marshmallows, tell stories and you can also watch an outdoor movie at the campgrounds. I've never done this but we're going to check it out next time.
CHARACTER BREAKFAST - We have been to Ohana's and the Crystal Palace. We loved Ohana's they had the best sausages I've ever eaten in my entire life. We went two years ago and I still talk about them! They also have Mickey Head waffles, not all of the restaurants do. And those were so good!! They bring the food to you on a huge skillet so you don't have to get up unlike Chef Mickey's and Crystal Palace where they have a buffet. I liked this better as it was still all you can eat but only your family was touching the food and you didn't have to get your fat a$$ back up for more.
And finally, a romantic idea would be to get some Fairy Tale Cuvee from one of the gift shops (I know it's at the Boardwalk) and some strawberries or other romantic snack and a blanket and sit on the beach of the Grand Floridian watching the fireworks!