You can buy training weapons if you are on the mood to do so. Training with these weapons can be pretty safe and effective.
Another thing to do is to spam occasionally spells in order to utilize the whole mana from time to time while inside of a Depot with the mana regeneration bonus activated. Most of knights do not train magic level, but that's a thing to do if you are looking for options.
Finally, you can sit back near the ocean or a river on Tibia and practice some fishing from time to time. Another semi afk activity option is going to some occasional readings on libraries spread over Tibia (some of the books are pretty interesting).
Edit: Oh, the Black Knight on Venore is quite fun to face and can be even a lucrative if you are lucky. You can sit near his respawn and check his presence from time to time (usually it is a very slow respawn, bit I don't go there it has several years)