Working out some game problems/thoughts

So now I am in the stages of putting this game together, and working out what to incorporate exactly and general mechanics there are a few things I need to iron out so to speak.

 First of all, during class learning about AI, I started thinking about having more of an obstacle then just jumping platform to platform. So in my original game idea I wanted to have boulders falling and sliding down the platforms knocking the player off. Which, is a good idea, but, now I have a premise and such, where are these boulders coming from exactly? I either need to come up with a reason for them, or just ditch them.

In addition to or just instead of the boulders, whilst watching the AI lecture, I decided upon having an actual enemy. Now, what would fit in hanging around up in the sky? Birds. I have birds flying around. Easy. They fit, I can do that.

Now, my game so far, has no health or lives in theory. Certainly no health, the birds knock you off the platform, simple as that. The idea is that you are trying to climb higher, them knocking you off poses as a threat to this.

So, lives, do I want that? If you fall off a platform, does the player just die? Or maybe lives is a good idea. Then I have a collectible.

I need possibly one other good one, or a trigger of some kind.

I have been given a few good ideas -

-balloons
-pogo stick
-trampolines
-jet packs
-hat with propellor

Need more thoughts on this.