Parks alone? Yes.
All my credits at Six Flags Great America were by myself. It's quite dull, since I hate being alone. Though it's nice to be able to pick exactly what you want without complaints, waiting alone is terrible for me.
Riding alone? Yes, in both ideas.
I've ridden by myself on a couple rides, mostly in odd groups. And besides testing and going station to station on Skyride, I've ridden Wicked Twister completely by myself. Hearing absolute silence is creepy as hell. I'd love to Beast, at night, by myself. That would be awesome.