Wow. If she's your girlfriend, you shouldn't need to ask people what to say. Say what you think about her. Tell her what she means to you. If you can't think of anything on your own, you probably shouldn't be together. Don't worry if the things are sappy or cheesy, girls find it cute.
Sing to her: "... Dear old Dad, God rest his soul, he'd wake up I'm the morning and smoke a bowl, smack my Mom upside the head, 'Make me a sandwich' is what he said yeah ..." - That's What Girls Are For (Steel Panther)
Sing little verses you have made up to the tune of a familiar song. Of course the words in the verse will be about how awesome you feel about her. Then you don't need to say anything at any given time, just sing your little rhyme when ever you feel like it.
Tell her how beautiful you find her. Tell her how she makes you feel. About the sweetness of her character, things like that. You shouldn't have to think too hard to come up with reasons that you are, and were, attracted to her in the first place.