I want them to fight animals, undead and maybe even demons if possible.
The theme is mostly dark and dreary foes.
Are just the foes dark or is it a theme in your whole world?
If that is the case then kidnapping might be common and the rangers encounter could be with some peasants whose child has gone missing. It needs to be tracked down though a wilderness. Add a few brave people to help (e.g. the boys father and his brother, armed with pitchforks) and you have an instant mini goal with some NPCs as a buffer.
For the cleric you may be able to do something similar pacifying a graveyard. The local priest or some of the family of the people who are burried there can help the their loved ones find eternal rest. This might involve turning them, or just keeping them at a distance while some rites are performed to clean the source of evil. This makes a great way to have an encounter without a fight.
The fighter on the other hand is build for fighting.
I think that YogoZuno 's answer is quite good. Give the fighter something to easy kill. Adding an NPC or two or making the enemies quite slow will help. (Slow in case you have bad rolls and need to run away). A nice tactical map might also help (e.g. one narrow point though which a few monsters must pass, and which can be blocked by the fighter. This will teach him that it is not just about hitting enemies, but also about placement on the map).
As for the sorcerer: Charisma 19. That will come in handy with a nice political mission (e.g. convince the major of a neighbouring town to aid another town or to convince a group of NPCs that they really want to do something else. Which, unless bungled, is yet another safe no-combat encounter. No risk of dying.