Ahh, well off the top of my head...
Why not make it so that when you press on a mario, he jumps, but he doesn't fall off platforms. So he would walk back and forth on a platform rather than fall off the end, the only way to get him off a platform would be to make him jump off the end.
With that, you could make a little puzzle game, maybe have the screen scroll, but the idea being that marios start at the bottom of the screen, and have to get to the top, and all the control you have is to make them jump at the right time. The marios could get further up with the platforms, and the strategy would be to see how fast you can get them all up to the top. Like to save time, the player might get all the marios to a certain point before moving them one at a time to the exit, rather than moving one at a time.
Not sure how the engine is for scrolling or level design, but it might be a nice little project to try out, maybe even try putting your own graphics in - or go raid the 2D graphics buffet, there's lots of cool stuff in there that would suit the DS.
We're going down... in a spiral to the ground...
