Side missions will basically deviate you from the main story and completing them will not affect the main mission in anyway. But they are always fun and its good to experience something different in the game. Moreover, they provide you with loot which makes your journey a lot easier. There are tons of side missions in borderlands. Every region has its own and you should checkout and complete. Some of them are explained here
Childhood’s end – The writing is strong with this one. This quest gives you an insight of Angel’s childhood, taking you through Konrad’s Hold into Tannis’s lab and how Tannis got the powers. The player goes to memories of Angle in order to understand how they operate so that he can get his water purifier fixed in Rolan’s Rest which is up to Vault Hunter and Tannis.
The Droughts: Powerful connections – In this side quest, you will earn $225 and skin as reward. What you need to do is to ask Marcus about his Munitions Vending Machine in Drought region in order to start this quest. Then follow the yellow cable, roam around and find out how to fix it. You will need to kill Skag in order to get his spine. Then use it to fix the machines.
Out of body experience: AI Core – The reward for this side mission is a talking shotgun. In This the AI core makes you believe that he does not want to kill you (but he does) and you start to find a body for him to live in. But then you put it in a radio where he cannot kill anyone.
Sanctuary: Just a prick – This quest will be unlocked when you will complete the main mission. You will be rewarded with money in this quest. You just have to collect empty hypos for Tannis and they are located in different places. Collect them and place them in the cart and Voila! The mission is done.
Ascension Bluff: Head case – Speak to Vaugh to start this quest. You need to grab the head and go to nearby shed to access simulation where you have to collect memory fragments then you will have to fight the enemies and exit the simulation. You will be rewarded with random weapon and $473.
Eridian proving grounds – This side quest contains six trials but can be started only after the main mission is completed. Each trial will has different rewards and gauntlets with enemies and no death runs. It is perfect for end game content and can be played over and over again.
Swamp bro – This is the most epic side quests of them all. In this Chadd needs Vault- hunter to run through the swamp with him. He wants to fight with ravagers and then he has to get an elevator which launches him into the sky, making the quest more interesting.
These are just the tip of an iceberg. Borderline has many side missions to offer which will keep you addicted to it.