It is true that "there are other things in the back of my mind" as you put it. But the only reason they are "on the back of my mind" is because I didn't have time to get around to them. They are not "worse" than the ones I started working on. Rather they are simply less obvious to me, at least at this time. So I guess it would be a shame to "waste" them on a graduate student if my original plan was to do ALL of them on my own.
Let me put it more concretely. I am trying to do interpretation of quantum mechanics. I have several approaches, each has its own plusses and minuses. Since I don't know how the world REALLY works, I think the way to be "balanced" is to explore many different answers to the same question in parallel. Thus, even though they are DIFFERENT theories (in fact, contradicting each other), in my mind they are part of the same picture. In fact, there are some of the common features they ALL share. So I find my work incomplete until I done ALL of them. And if some of them will be done by grad student I would never feel like I have done the entire thing.
Now of course the grad student never said that I should specifically supervise him on this. In fact from what he said he is quite open to anything from quantum gravity to cosmology. But you see, I can't think of any research topics outside of what I am actually doing. I don't know what cosmology project to give him, as i don't know much about that field (and the latter is what limits me to give him some of my own projects).
I geuss the other option wouuld be to give him a project on my ex-favorite topic, that is, causal set theory. I know the area because I USED to work on it, and I won't feel AS sorry if he does something there since I changed my interest and am not planning to go back there any time soon. But I guess if he does something nice there I would be SOMEWHAT sorry since then I would regret that I changed my interest. But still not AS sorry as if he does interpretation of QM, so in this respect it is better. But on the other hand I would not be as good of an advisor there as I would be in interpretation of QM since, obviously, I changed my research topics for a reason.
Anyway, what do you guys think?