As far as easy ways go, you could mod the physics, and there are a number of "tweak values" in a bunch of the weapons that can radically change their behaviour with little-to-no effort. Umm, beyond that, there's NPC creation which is my favourite, if a little challenging, and engine modifications which are probably the most ambitious, such as GIR's 45 degree slopes. UI like the energy and health system I'd probably file under engine mods, but of course the difficulty always depends upon the desired outcome. The hardest part of it would probably be finding where it is and what it does, and then how to change it without breaking stuff.
Anyway I'm going to sleep for now.