Hi, welcome back

Slayer is really good for money making and mostly semi-afk, but it does take a very long time to train. Once you get to 60 and unlock the Karamja volcano slayer dungeon, there is a good spot with 4 black demons which you could probably make some good money just chilling at.

I can't think off the top of my head what the best money-making spots would be without training slayer. Wild dragons are pretty good, but less AFK because of the fire damage they chip away at you with at the start of each combat, and the obvious danger of being in the wild.

Anyone got any other ideas?

I thought Slayer was an OSRS skill, not classic? Sorry... it's been a long time.

Kill the revenant boss in the members dungeon and abuse the bug where you can't be attacked at the border by other players and you can break the mechanics of the boss making it very easy, probably best GP in game, tied with or better than having 99 slayer but there are zero requirements. Staff won't do anything either if you choose to do this!

Do the super engaging and interactive piece of content over and over and over again, the members mini quest, get yourself to 99 slayer, 99 runecraft and agility without ever doing those skills while making absolute bank, you'll be popping out tridents out your butthole, an item that's basically better than draco whip in most situations but is 100x easier to get and 100x cheaper. But it's fine you'll get so many of them it will be worth it!

TLDR: Kill the revenant, do the p2w members quest over and over again, rinse, repeat, you'll be rich in no time. Grinding 99 slayer is 600-1200 hours, spend that time abusing Rev and members quest I guarantee you'll make triple what you could make by playing the game normally how it was supposed to be played.

Joke ting.