Farming trees could somewhat be considered AFK, but it's more of a passive training method.
There is an AFK farming method though, which is within the Chambers of Xeric.
You can train by planting and picking the herbs within the Chambers, and at a high farming level, you can get around 50k XP per hour, with AFK intervals ranging from 20 to 40 seconds at a time.
This method is decent for Ironmen as well, since you don't need to collect seeds, and it's particularly great for Ultimate Ironmen.