Side missions, also known as side quests, are optional quests which have their objectives marked "[Optional]" in-game.
Side missions must be completed in the map they were assigned. If you leave the map (or even log out) the mission's progress will be forgotten when you return, and must be started over from scratch. If a quest requires you to salvage a resource, you must do this after the quest starts; simply having them in your inventory before the quest won't count.
Every day at 3 am UTC, side quests are reset, and random ones are chosen by the server to be available for the day. These ones that show up are the same for everyone on a specific server (although if too many people are on a specific map and it "splits", the new map will have different ones available). As the ones that appear are completely random each day, it's possible for a specific quest to appear / not appear for multiple days in a row.
It is possible to change servers and thus do most / all of the quests in a single day. However, since which quests are available on which servers is random, it is possible for a specific quest not to be available on any servers. Some flags are merged with each others, making them redundant to check, here is a current screenshot of the servers: Servers List.
The easiest way to tell if a zone has an active side mission is to use a compass. When used, the compass will show what direction the NPC is located in, and if no NPC exists, nothing will happen (which indicates it's not active).
Location: Follow the river at center of map north along the west side until you find her on a wooden dock.
Objective: Talk near apple tree, salvage 3 chocolate.
Infecteds occasionally drop Biscuits, which have a low chance to give chocolate. It is advised to find other chocolate giving sources such as strawberry cake and chocolate boxes, both of which can be found in vending machines as well as some set locations (it's worth leaving the area to find it and then restarting the quest when you have good chocolate giving items). Go north past Tiscornia park and northwest to the memorial statue. Talk to Sophie.
Zombies will occasionally drop rope or wooden planks (mostly rope). Pick up any Guitars you find, as they give both wood and rope. There are some set rope locations nearby: one is to the west, north of Emett's side quest building. Another is to the north east, in the house south of the pool, next to a desk (there is also a guitar in this house). There is also a rope along the northeast of the pond by Amber. Another rope can be found right beside one of the broken-down cars by the BiBi-Too Café (which is at the left of the gas station).
Infecteds will commonly drop Lighters (1 fuel), and occasionally drop Gas bottles (5 fuel). You can also find fuel, above the gas station, to the right of the gas station, and a gas tank near the entrance to the terrace gardens secret passage.
Reward: 2400, 6 Rep, Camera (random good+ quality)
Location: Go north from spawn. Upon seeing the lake, go west along the bottom of the lake until you see a house with a white flooring. Go around to the left side of the building and go north past the beach. When you see the bike, go east and you'll see Abbi Webster.
Objective: Take pictures at various locations.
Go north through the wrecked house, and take a picture. Next, follow the path east past the wrecked house and take another picture. Continue east and take another picture. Return to Abbi; she is just nearby.
Best way to do the quest is to bring 2 Candy dispensers or 10 fruit Juices. There's one candy dispenser in the playground right near the NPC, and another one in the southern tool shed near Matt Sanders
Juice can be found in vending machines and as mob drops while doing this quest. Check the convenience store for vending machines, it's east of spawn, just below the Farm Secret Passage, where Gabriella Campbell is located.
Location: From the start, go north and turn east onto the wooden path, following it until it ends, and continue east until you hit a wall, then head north until you see an east path connecting to a small parking lot.
Follow the south-east path until you reach a dorm building on the east edge of the map (entrance is on the north west side) and travel to the south west corner room, search the dresser for pet food, and feed the hamster. Go to the east side of the building and exit out the hole in the wall and talk to Glenn.
Along the way to the casino in the Egon search quest, there is a golf course (to the west of the casino). Search for golf balls scattered in this area. Return to John, who has moved into a building at the southern end of the golf course.
The best source for components is Screens but Remote controls will start appearing frequently, electricity can be salvaged from all kinds of battery items (especially Laptop battery), and the best source of metal is probably finding an Industrial metal piece (there's a set location for one in construction area). You can find both electronics and batteries in the room NE0 is in, but if you need more you can check out the cyber cafe south west of the pool area and south of the super market entrance (the one that connects to the SP - Freezer).
Location: Sitting in the middle of the diagonal stone and wood bridge, north of Jordan.
Objective: Salvage 10 salt
While on this quest, infected will have a chance to drop crab, which can be salvaged for a chance to get salt, and you can also fish in the nearby fishing spots for various fish. Talk to him again when done.
Location: Next to the unlit fire near the second bike adjacent the taco house.
Objective: Kill 20 infected near house, inspect water tank.
Go northwest past the greenhouse to his house to kill 20 infecteds. Go back and talk to Dr. E. Pollick. Afterwards, go back towards the bike, down the slope and northeast to the water tank to inspect it (Alternatively, southwest to the museum and swing around to the north end). Return to Pollick.
Infected rarely drop tools for you, so scavenge in the railway secret passage and around the map. You must salvage the tools after the quest has begun or it does not count. Talk to Joshua when you are done. Go to the front of the museum into the toy shop, and take a dinosaur plush. This time killing infected will drop green bags containing them. Return to Joshua. He won't take your tools, but he will take a Dinosaur plush.
After speaking to her, you first you have to read the diary next to her and then she requests 10 Fish and 5 Meat. Infected will now drop gudgeons and chicken sandwichs more often. Collect the resources this way takes a lot of stamina and time. Alternatively the surrounding buildings can be searched, especially the Diner in the West. Now return to Tasha.
Location: At the very bottom of the map (just to the right of the bottom bike and campfire) between the small pond and the adjacent building.
Objective: Reeves will give you a Flat battery; interact with the nearby generator to use it. [Return to Reeves.] Deal 750 electric damage. [Return to Reeves.]
Note that the voltmeter quest item does not deal electric damage and you'll have to find some other way to deal electric damage. Best bring a taser for this quest, or if someone else already completed the quest, ask to borrow their taser. Microwaves deal electrical damage as well if you can find one.