Both of the suggested methods involving Summoning ingredients supposedly work well, with the latter actually helping your mining skill in the process.
For a longer-term strategy though, you may wish to go with Slayer. I'm currently 93, and for the most part my money is from Slayer and the related things I've done through the skill. Clues, drops - they all add up. I have gotten some of my money at Barrows, God Wars Dungeon, and other such locations, but I never get *that* lucky. Never have had a hilt drop, or a split worth more than 2.3M (that I actually received

).
You could also train your Range at the Aviansies, though you'll probably want to be higher range to start.
If you're going to remain F2P, Woodcutting is by far your best option. That is, unless you happen to be quite lucky at Bounty Hunter
