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Mining - Power mine iron with a rune pick in places where there's more than 1 iron rock to 90 or so, then coal is best.
Rune crafting - this is slow unless you use one of the major
companies. Go to world 16 and craft there by giving the runners 2k +
essence back per run. This drains money fast if you are under 55, but
once you get over it's a good way to make cash. If you don't want to do
that, your best option is to either use the fire altar for members, or
use the earth altar for free.
Combat - if you are member, use pest control. If you are free, get
to level 60 or so combat and then train on flesh crawlers on the 2nd
floor of the security dungeon.
Magic - get to level 25 magic using combat spells, then use the best
teleport you can use until level 55. Then high alch until 99.