Money in Runescape can be very easy to come by, or very difficult, depending on the method you use. Some prefer grinding skills like fishing, mining, hunter or woodcutting, and some prefer luck in combat hoping for high level drops like spirit symbols, draconic visages, godsword shards and hilts, and other high-level items. One method of guaranteed money making often overlooked by players, both old and new; kingdom management.
Now, be warned, this does require that you complete a quest (oh the horror!), but if you manage it right, that small amount of pain will be worth the rewards. This isn’t just a quest that has a one time reward upon completion, it’s one that keeps on giving. I can assure you that if you maintain your kingdom, you can make up to 3 mil depending on what resources you choose and their price at the GE at the time.

What you’ll need:
≥1 mil. of initial investment
Throne of Miscellania quest completed (guide)
Time and a little patience
1 visit a week to collect reward, restock coffer if needed, and maintain approval rating
Optional for more resources collected: Royal Trouble completed (guide)

Once you have completed Throne of Miscellania, you are named regent of the kingdom, and have the right to manage resource gathering throughout the land and then collect said resources. You then have access to six different resources that can be gathered (technically 5) being fish, maple logs, coal, flax and herbs. Completion of Royal trouble adds three more: mahogany, teak, and farm (you can also do a mixture of the 2 woods). I highly suggest that you also do Royal Trouble, as it adds more workers to the pool to the point as to where you can effectively collect 2 resources.
This does bear many advantages as resources can be sold immediately on the Grand Exchange or can be hoarded for use in skilling (e.g. coal for smithing, herbs for herblore; fish for cooking or combat; wood for fletching, construction or firemaking). Now do remember, these items are collected at minimum effort from you, as all that is required is the one-time quest and maintaining your approval rating (which only takes about 5-15 minutes a week). At current prices, the two most lucrative things to collect are coal and herbs respectively because of the necessity of large amounts of coal used in smithing and the necessity of high herblore to be able have the competitive edge in pvp with extreme potions.
Also, since the resource gathering is essentially npcs doing work for you, two things happen: 1; you don’t gain any xp by gathering and 2; you will receive gems from mining, seeds from herblore and birds nests from woodcutting, adding additional revenue to your already growing pile. The kingdom with withdraw a max of 50k per day regardless of approval rating, so it is recommended that you always try to keep approval rating at maximum to take maximum advantage of your money.

To prove my point, if we take my personal average of 2.5 mil per week by selling everything on the grand exchange, and subtract 50k times 7 days a week (i.e. 350k per week), we end up with a total profit of 2 mil 150k profit. I am making essentially 614% profit per week on my investment of 350k. To put this into perspective, in a real stock portfolio 7% growth per year is good and 20% is amazing, and to get 100% or more is unheard of.

For quick money making for very large amounts, combat is still best. But, for the slow and steady that requires almost no work and a reasonable investment, the benefits clearly outweigh the “hassle” of maintaining it and doing the quests.

This is my current setup for my kingdom, with most on herbs and the rest on coal mining. It usually yeilds an afforementioned 2.5 million gp per week at optimum production.