How to tell a girl that you want only to stay as friends?

I laugh to give her an impression that maybe I am bored or un-interested, guess it's doing the opposite. I saw her today, and we talked here and there, then she suggested that I cuddle with her. So I'm like "Heh. Sure?", and she was putting her head on my shoulder. Pretty damn positive she likes me now. Like, super-positive.

Stop leading the poor girl on.
The longer this goes on, the more times you're like "sure", with or without question mark, and semi-cooperating with her attempts at closeness, the worse she is going to feel when you finally do tell her (or she finally realizes) you never wanted any of that.