heh, you know, if you have that really nice water effect as togglable, then it should fit in alright with the videos, Although I couldn't say for certain. Meh, whatever you do is fine.
Also, make sure that the mini games have to be unlocked. That's one of the things that makes them so appealing. Like say, there was an item that unlocked a specific mini game if you collected it in a level. If I'm correct, then there are four mini games planned, right? Time Attack, Cart Racing, 2D D&C, and Snake/Dance game. Well, you could hide the items that unlocked the mini games in four of the levels, specifically the second, third, fifth, and sixth (the first level and the train level wouldn't make much sense to add one to).
If you were to set it up that way, not only would the mini games themselves expand the play time, but so would the search for them. It would even mean that you could limit people playing the demo to only being able to unlock one mini game, without adding extra constraints.
Of course, if you were to hide the mini games inside the levels, you would have to make the levels repayable in case someone didn't get the mini game token (or whatever) the first time around. I really think you should make the levels re-playable, at least after you beat the main game... It's frustrating to have to start a new game if I want to re-play a level...
Edit: Now that you'll be getting D&C published, do you think you'll
eventualy make D&C2?