Lee's a great ad for TT. And getting them started with some skill shots is a good idea.
I don't like new players learning on sion. I did so, myself, playing him the majority of my first probably hundred games - it set me back. His kit is so .... split .... re ap/ad, his stun makes you not really respect positioning, and he's just generally "easy" but not with much transferable skills.
Rumble is a great pick, and someone likely worth learning. Irelia, maybe. Xin is fun and requires some thought. The difference between a mediocre shaco and a great shaco is just silly.
Let them try carry you, occasionally. Pick blitz, and set them up.