You can hold onto her thumb, or wrap her pinky with your index finger. There is a whole language describing different levels of intimacy, comfort and attraction.
It warms everyone up to you as a friendly, confident person, and makes all kinds of non-sexual touching completely natural.
She was already ‘into’ you, so no fancy stuff required. In my years of experience working face-to-face with literally hundreds of diverse guys I’ve seen this problem in every single one of them (myself included! Many of these guys I only get to spend a short amount of time with - maybe I’ll see him in just three or four conversations with women.
Yet even in that incredibly short ‘sample size’ the evidence is obvious.
If you do something she likes, she might squeeze your hand to let you know.
If things are going really well, you might try to play with her fingers gently, or you can tickle her palm.