it's really simple.
- create a menu - add some options like particles, explosions etc.. and add checkbox to that.
- when selected - enable event group that controls particles, explosions or what not
- when not selected - simply disable - therefore no particles/explosions/etc.
ofc. you have to think this thoroughly. if you want texture quality and more, someone with more experience might answer, but i think you will have to preload/load all the defs (high, low, medium) and more stuff.. probably memory consuming.