loads of players seem to be wearing backpacks instead of capes. What are they and how do you get one?

you mean this?

 

 

 

 

 

 

 

http://tip.it/runescape/index.php?rs2quest_id=152

I believe the "backpack"s you are referring to can be 1 of the following 4 items: attracter and accumulator(dark green and grey colour) from animal, diving apparatus (tan brownish colour) from rfd pirate section or bonesack (purplish colour with bones hanging out from its mouth on the top) from rag and bone man

i believe your reffering to Ava's attractor or Ava's accumulator that you can get from Animal Magnetism quest. The attractor will generate an iron arrow from time to time as well as pick up your iron arrows you shoot and put that back in your inventory, the accumulator does the same thing, but with steel instead of iron.