Gameplay is the relationship between the interactivity and the rules.
I thought something along these lines when reading the first few posts here.
More basic though, in my opinion there is gameplay
and interactivity
. They are neither competing elements nor congruent.
Gameplay can be caused by interactivity, although it needn't be.
Interactivity can cause gameplay, although it also can just act on its own, like a toy.
Regarding SimCity being a toy or game:
I think of it more like a toy that is being played by more than one person. One is you, the player, the other is the computer. Even though the computer is throwing rocks in your way, with let's say earthquakes and stuff, its motivation is not to destroy you and win the game - it can't win, only you can lose - but to make your experience more interesting and challenge you.
Compare this to two people playing ball. Now, both of you could go on tossing that thing directly into each others hands, but it would get boring pretty soon (for me, at least). So maybe your partner starts throwing high arcs and other difficult to catch paths. That would spicen things up. But still, no one would be trying to win.