If you have a high combat level, there are many other ways, such as killing blue dragons in taverly dungeon. With a high slayer level you can kill many monsters that drop great items. As a low level, obtaining level 1 clue scrolls is an easy way to have an adventure and gain some money at the same time, though the 1 million might not come in one hour, you will enjoy your time training and treasure hunting.
Runecrafting: 2x Nature runes 950k/hr Hunter: Dragon implings 300k/catch 600-1200k/hr Hunter: Kingly implings 1.4m/catch 1.4-2.8m/hr Slayer: Abyssal demons drop Abyssal whips for 3m each about 1 whip every couple hours Slayer: Dark Beasts drop Dark Bows for 1.1m each Thieving: 4x Master farmers 900k-1.1m/hr
However, these all require extremely high levels in their respective skills. One of the only ways to make mills an hour is with merchanting but this requires massive start-up funds.