I made an endless runner. It's pretty standard really.
Purple orbs. Collect enough and you'll speed up and change outfits.
Helmet. This gives you armor so you can take a hit.
Fire. This will burn you up.
Fireball. The same, but in the air
The orbs were coins, but the color matched the fire too much so I changed to the purple orbs.
I notices a glitch where I could walk through the moving enemies since I didn't made a proper collision check, but I thought it was neat so I just made a sound effect when it happened instead of fixing it. I think you only have to do it once to get everything.
I'm sure there's problems with this, but I wanted to finish something before I started acting ambitious.