I like the Golden bullets idea! Scatter 3~5 of them depending on how big the level is, and they must use it in that level so that they don't accumulate the Golden bullets to use in harder levels
Edit: or how about some kind if "health extension" item? after collecting this item, your HP is full and gain additional 30% health or something and it expires after certain time has passed (also you can't heal above regular 100% HP). Or something along that line
Or if it's possible, an item that grants faster shooting rate for certain time.
Another idea, lots of puzzle pieces scattered across the whole game, and if you collect all of them, you unlock a secret level (and make it survival mode) (again, only if it's possible