What is a word for "slightly smile with embarrassment"?
Example:
"You look great tonight," I said. She ______ed, trying to hide her face from me.
Although it isn't a smile, the word blush works perfectly here:
To become red in the face, especially from modesty, embarrassment, or shame; flush.
"You look great tonight," I said. She blushed, trying to hide her face from me.
[TFD]
"You look great tonight," I said. She smiled, bashfully, trying to hide her face from me.
If you can get that down to one word be my guest. But I say a bashful smile is closest to what you want.