a mushroom, oh and some turtles, and a plant coming from a pipe!
tbh, just look at any game, or your surroundings,
"good artists copy, great artists steal"
"The worst thing an artist can do is try to copy someone else"
I would say, have a look around at the other games and draw ides/inspiration from them but don't attempt to copy or rip them off unless of course you're planning a fan project or parody.
Really though, the enemies and such would depend on your game and or story, what is going on. What are the objectives and or goals of the levels?
Until you answer things like this and how your game will play you cannot simply come up with enemies or how you will encounter them on the spot like that, well you might be able to but it would make your life easier thinking out other things and incorporating those design elements into the process of creating such enemies.