Rune making on depots, just grab food, daily streak full, maybe some rings and keep doing runes, isn't the best profit of the life but is completely harmless so you wouldn't have downcomes as dying or so also u can just put your chart on-dp,time-up for the max mana, then just checking timely to rune and going afk again.
Fishing for northen pikes, this one highly depends on demand, but on active servers (non so old) you would find ppl interested, the idea is simple, you gain access to nibelor, trap the mobs with parcels and then just fish the different holes of the place without suffering too much, even if u dont trap the mobs it is almost impossible to die after lvl 100 there.
Doing quick task, like warzone tasks where u can just jump out the place if you get busy both warzones task follow that pattern, aren't as chill as that u can just turning the head around and not die, but leaving the place takes no time so aren't hard to deal with.
As said by shawtay u can also try to get some pearls from opening seashells daily (sea serpents, feyrist, etc), aswell u could get a dwarven ring from crates remember u can just open 1 of those daily, so isn't a huge task but east and chill.
Doing totally harmless task: like rottin wood one, venore citizen ones, etc (there are others but atm i don't remind them).
Doing as merchant: either you flip stuff on market or you simply buy low price stuff to be sold later on npc (djinns, rashid, yasir, esrik, etc) both require larger ammounts of money to keep selling and buying stuff and the last one require access but is worth on both case if you have the time for taking care of passive incomes.
Hunting harmless stuff, lets say you have 1 lv600 and 1 lv200, but u are doing nothing the whole day and u can't hunt hard stuff with your main, then u just hunt things with your second chart (to prevent wasting stamina) which are considerably weaker than what you usually hunt so then won't kill you if you need to be afk for FEW seconds.
example: hunting amazons, mutated tigers, vampires (only vampires), and all the mobs that provide imbuements, you wouldn't die if you have enough lvl so u can chill-hunt them.
Now, if all this isn't enough for you, i have some others TRY-HARD TASK that could be tried while being busy, but ofc u would need to forget the word "semi-afk" in most cases, i mean, u can do such activities and then get totally AFK without being harmed, but while on the activity most of them need 100% of your focus.
1- Looking for bosses all-around tibia: thankfully tibiabosses have several useful guides/tools to aid your enterprise, just walk around tibia and check those specific places where a boss could appear, if u are lucky u may find one, if u are really lucky u could be rewarded with something worth, u spent almost nothing with it and nowadays with a lvl 200 is almost harmful.
2- If that's not enough, then maybe u could try Daily-fixed respawn bosses: a lvl 200 have plenty of bosses to try daily or even "grind them", yet my recommendation is a lvl 400, but 250+ were good too, the higher your lvl the most bosses u can kill! and the easier the task is but i can relate that a lvl 200.250 can deal with most bosses: lloyd, urmah, leyden, tenebris (require good team), Gold token daily rotation (require lvl 250), oberon (require only 1 high lvl ek friend or team lv 250), scarlet (3-4 player is good 250+), drume (i haven't done this but i guess u can do as 250 aswell), dream court bosses, etc etc etc most of them require team, only oberon scarlet and drume can be done alone but is REALLY risky at low lvl.
and that's not all! there are bosses with regular respawn, graven danger bosses, minor bosses on falcon bastion, urma mini-bosses, etc etc,
the idea is simple: u join, beat em' all, u leave, get afk right after. or simply u kill them get afk for less than 5m (they respawn in that time), then focus again on killing them, with some luck it would be REALLY rewardable.
3- The ultimate Hunting/exploring task: find fiendish. personally speaking this is the most entertaining activity on all tibia, specially if you are a lone-wolf either by desire or by the circumstances, this activity require knowledge of tibia, some quests, a lvl 200-250+, several access all the tools on your hand and more, i mean if u want to do this regularly and don't get stuck with "easy" places, but in return this is:
A) Almost harmless, the only way u find strong enemies is if you are looking for them, thing that u can skip if you want, even if u know something is on a really hard place not neccesarily u would go in if u dont want, right? but of course first u would need to improve your "finding" skills to know where is that mob, don't you? (maybe this find fiendish messages and this find fiendish tips pages would aid you)
B) Considerably Rewardable: either if you are a high lvl or a low lvl (200-) finding fiendish mobs is one of the most "easy" task u could do on tibia that provide a "decent" reward, remember that such rewards depend on market, but seems like those "goods" (slivers, exalted core) have reached a "base price" which is worth. (remember always exchange your dust to maximize your gathering/profit)
C)Almost Semi-afk: this is probably the most interesting thing, if you are looking for trivial/easy mobs, in most case u would find them in easy places, even some "medium" mobs follow this pattern, so u can simply chill out while spending some time finding, but always take into account that your tibia knowledge plays a really important papper here, even if you are looking for a sandcrawler, u shouldn't "chill out" in issavi, don't u think?