Final Fantasy XV is filled with side quests in different locations. If you haven't finished all the side quests with Cid, here's a complete list plus its locations and XP gains.
Cid Sophiar, Master Mechanic
For this, go to Cid to get this side quest right after you finish Chapter 1. You'll need to find metal scraps to upgrade your Drain Lance. The metal scraps are everywhere. There's three in Hammerhead, one in Longwhythe Rest Area and also at Crow's Diner. This will give you 500 XP more.
A Better Engine Blade
This is an easy task. All you need to do is find Rusted Bit, which you can find as you journey along the game. Take them back to Cid so he can upgrade your Engine Blade. You'll gain 500 XP after this.
A Better Engine Blade II
To upgrade your Engine Blade, find a glass gemstone which can be found in the building next to the street in Lestallum left of the Nissin Cup noodle truck. There's also one down the stairs around the corner on the East side of Old Lestallum, and another one in a lone cabin north of Cape Caem. You'll get 800 XP more after this.
A Better Engine Blade III
After you upgrade your Engine Blade for the last time, you'll be able to get 1,000 XP and an Ultima Blade. But before this, you need sturdy helixhorn, a rare item which can be found by breaking the Duplicorn's appendage. This can be done during the Lestallum Bounty Hunt which requires a Hunter Rank 7.
Better Drain Lance
Defeat a bunch of coeurls and get their whiskers. You can find them in areas like south western Duscae near the Taelpar Rest area and in northern Leide at the Keycatrich Trench. Give the whiskers to Cid so he can upgrade your Drain Lance. It will get you 800 XP after this.
Better Auto Crossbow
When you upgrade your Auto Crossbow, you 'll get 2,000 XP. To do that, get some cactuar needle by defeating a cactuar first. This is very rare and can be found South of the Vesperpool where there is a curved road before the dirt road starts on the Western Road. You can also get one when you kill a Slactuar which appears every morning in the Northwest of Hammerhead, North of Fort Vaullerey, north of Alstor Slough, and East of Steyliff Grove.