Hi Nathan
You're right on the nodding heads. A local pair that I've known for around 6 years now always do it when they see me. Their cygnets copy them too, so last year at one point I had 8 bobbing/nodding heads coming my way. You may also notice that they snort (a few quick snorts in succession) when they are happy, usually just after being fed, which seems to be something akin to our "thank you, or, I like you". They only do it in certain situations and it definitely seems polite/affectionate.
My favourite is their "elephant" esque rumble they do though that is nowhere near as common, at least not with the 2 I am close with.
They also recognize sound. I can call the swans over to me from anywhere inside their hearing range. As soon as they hear they start looking until they see me and then they come. I always use the same word, the name I call the cob, but they all answer to it. When I find someone I can trust with them I want to try an experiment. Get them to call using the same word. I'm very interested to see if it's just the sound they react to, if they look, if they come when they see where the sound is coming from and would they come out of the water when they see it's someone they don't know as well. If they don't their response isn't just based off a familiar noise and suggests a far higher intelligence than many give them credit for.