Sometimes you can catch yourself just starring at your girlfriend without saying anything. Of course, if she could look into your mind, she would see how fast you are going through all the sweet things to say to her and almost instantly throwing most of them away: this is too naive, this is too girlish, this is too boring, this will not make her laugh, etc, etc.
If you catch yourself in such a position, just stop making any assumptions of what she would like to hear. Tell her someting positive, something that doesn’t sound too “clever”. Sweet things to say to your girlfriend in such situation would be: I love the way you look at me.