I decided to hop onto Runescape for the first time in what seems like forever with the intention of actually doing something. I went to Port Sarim and checked out this new sort-of minigame which is similar to kingdom management.
It is actually pretty cool even though it isn't something you can actively do. You probably want to go there about every 30 minutes to do stuff and then go do other things with your time.
Once you send your ships out, you don't need to stay logged in during that time. You can log out and do other things.
This could be considered high level content I suppose since it requires level 90 in either fishing, herblore, prayer, runecrafting, slayer, or thieving.
Apparently the rewards are really good as well and are for high-level players.
Have fun. I used to love thieving and that was my way of making money. But that was like 9 years ago and I was like lvl 70 on my first runescape account. I would always be in Ardougne at that silk stall or up in that one building getting those nature runes