Sorry for the typo earlier , she will be a little over 3. Is there plenty that someone that age can do? Dumb question I'm sure , but I figured I should ask. Thank you!
Yes.
I suppose the answer to your question depends on what YOU expect FOR her. Any child old enough to experience sound, color, and fragrance will experience the sensory stimulation of WDW. They will know the vibrancy of the colors on Main St, the smell of fresh popcorn, the sound of fireworks.
As to what she can't do, it depends on her height, not her age. Multiple rides have a height cut-off that fall in the 2-4 year old range. Even supposing your child is short, you could easily spend a full week just seeing characters, eating table service meals, and swimming at your resort - and never even see one of the many shows, parades or, ride one of the many attractions she'd be tall enough to ride!
Just off the top...one place for all ages is The Seas in Epcot. There is a Nemo ride, a show where children are invited to ask Crush the sea turtle questions, and many live animals to see! Plus, no surprise, a small gift shop. Groups of all ages can easily spend over an hour in that one pavilion!
I've been with kids of all ages, and they all benefitted. I can also say, of places you could take a young child, WDW is among the most kid-friendly destination of all.