Given that winning random enemy encounters does not make you rich (at least not directly) in Final Fantasy XV, completing sidequests is good way to earn gil in addition to other rewards. These extra missions are marked with a yellow "?" on the map, and once a quest is started, players can activate guide markers (blue "!") on the map to help figure out where to go.
To turn on the quest marker, select Quest in the main menu then choose the mission you're trying to work on. Below is a list of the sidequests I've found so far, grouped by area. Rewards are listed next to the quest title
Longwythe Rest Area
Gone Hunting - 500 EXP; 10x Hi-Potion
As soon as the Regalia is fixed and ready to go, Cindy will ask to deliver a package to the motel for her. When you arrive at Longwythe, speak with the motel manager to have them collect the package from your car. Speak to the guy standing outside the motel next and he'll ask you to deliver something to Dave. He is in front of the white shed across the street. Give him the envelope then go to diner and speak with the Tipster to ask for information. Now, activate the sidequest marker and go to the location. Grab the dog tags and take them back to Dave.
Galdin Quay
Kitty Catering - 600 EXP; Dragon Scales
To find the "client" for this mission, head to the docks north of the restaurant and speak to the cat. Noctis decides to feed it with fish so activate the sidequest marker and head for the fishing spot. There, you will be playing a mini-game. Fish as instructed then head back to where the cat is once you've caught something. Turns out the feline wants its meal cooked, so go speak with the Tipster at the restaurant and have her cook the fish. Feed that to the cat.