Here is the truth: girls love compliments. They are very emotionally sensitive and tend to be more affectingly sophisticated to words than guys.
They react like little girls whenever some part of their astounding attributes (especially physical) are noticed and loose control whenever it happens. They need a lot of attention to satisfy their emotional needs and giving compliments can one of the best ways to do so.
The following might be considered romantic things to say to your girlfriend, but at the same time can also be poetic, funny, Philosophical, silly, weird, etc, but I prefer saying that they are sweet.
Sweet things to say to your girlfriend:
Silly
1- You're so sweet. I might go diabetic
2- If I was superman, you will be my kryptonite
3- Thanks to you, I've fallen in love and I can't up
4- I wish I had glasses so I can see you with four eyes
5- You are my girlfriend because you're more than just a girl, you're my friend
6- A million dollars is nothing compared to the number hugs of I can give you
Normal
7- You are my best friend
8- Your voice is sweeter than honey
9- Our love is friendship, but set on fire
10- If it wasn't for you, there would be no me
11- To me, your smile is brighter than the sun
12- When I close my eyes at night the first thing I see is you
13- If I had a million words to say, they will all be I love you
Poetic/Philosophical
14- Without you, a second is infinity and with you, infinity is a second
15- Your name is the drumstick that strikes the instrument of my heart
16- If my love for you was visible, it would not fit in the space of the universe
17- When you're away, I feel 50% empty, when you're with me I feel 101% complete
18- I think about you every second of every minute, 25 hours a day, 8 days a week of all the 13 months of the year and in the undefined infinity of all eternity
Rhymes
19- Without you am just simply blue
20- Your voice sounds better than all my favorite songs put together.