I think this usually works very well for exploring games, some of these could also work in Super Monkey Ball:

-Use one object alot in the level and make variations to make it more interessting.
-Have a theme (This could be that object, or a mechanic)
-Have depth, make it so that people get curious and that it makes them want to explore. (like little secrets that give you little rewards, significant objects that stand out from the others).

For Monkey Ball:

-You could look up some interessting geometrical shapes that have special properties and make use of those (for instance: the Brachistochrone curve, look it up there are videos about it)
-Make something based on a real object (like a rubiks cube)

What i usually do is just start modeling, goal-less exploration. Just experiment with different things