Me and a friend have been making a side scrolling platformer for a while now, but we haven't added much in a while. After a period of rapid development, our ideas have just run out. You play a generic astronaught as he traverses first a robotic city, then a dark cave on a rocky planet, and finally grassland on earth. The levels are always a fixed width and height, and are generated randomly. You play for 4 levels in one zone, then the setting changes. You pick up jewels that can be spent on opening locked boxes you find throught the game.

So far the game includes:
1 and a half finished zones
13 different enemies
10 different useable weapons
15 items to pick up

The games controls are the arrow keys to move, Z to jump, X to fire weapon / throw object, and C to open lockbox or pickup object.
You can see everything so far here.
The game is intended to be quite hard, by the way.

We need 2 more enemies for the cave zone, and the earth zone hasn't got anything in it yet. What should we add?