Merlin only uses his magic for emergencies. At least that's what he tells Gaius, who doesn't say anything but raises one eyebrow, giving him The Look. Merlin only uses his magic when he has no other option, like bandits attacking, or magical creatures, or evil sorcerers trying to kill Arthur. Except for when he doesn't.
Sometimes there are days that everything just feels so alive, he can't help but add to the celebration. The flowers in the vase end up looking a little more fresh, the colors a little more vibrant. The music from the courtyard wafts in from the window when it's just a little too far away to be heard. One of the maids Merlin passes in a hallway might check her apron pocket later that day to find a tart she swore she had already eaten.
Sometimes there are days where everything seems to go wrong and Merlin needs something, anything to make it just a little better. When Arthur expects Merlin to have polished all his armor, sharpened his sword, fixed his chainmail, and have written an important speech by morning, some of those tasks get done with a little magical help. When Merlin still can't breathe right from the broken ribs he got from getting slammed into a tree by the latest evil sorcerer and trying to muck out the stables on top of that makes him feel like someone is stabbing him in the side, he might enchant a pitchfork or two to make it go faster.
Sometimes there are days when Merlin is annoyed with Arthur but can't tell him directly on pain of being thrown in the stocks- again. Arthur's bed might be unusually hard and uncomfortable, the result of a ticked-off Merlin tired of having shoes and pitchers thrown at his head. Arthur's armor might not fit quite right, leading to some uncomfortable chafing in uncomfortable places the day after Merlin's warnings went unheeded for the upteenth time. Arthur might wake up every half an hour during the night, swearing up and down that the cricket's incessant chirping was even more obnoxious tonight, only for Merlin to give him a confused look and ask what exactly Arthur thought Merlin could do about it.
Sometimes there are days where Merlin is fed up with the small injustices of the world and decides to do something about it. The stable boy who was slapped by his master for something he didn't do finds that the next time his master raised a hand against him he suddenly forgot what he was doing. When Gwen is assigned to serve an arrogant, entitled visiting noble who gave her one too many lingering looks, the noble's embezzling is discovered by the court due to some important documents escaping from the locked chest where they were normally stored. The little girl and her younger brother begging on the street discover a decent sized chunk of bread and cheese wrapped in cloth that certainly hadn't been there a moment ago, but they aren't about to complain.
Merlin swears up and down that he only used his magic when absolutely necessary and doesn't waver under Gaius' scrutinizing gaze. After all, he isn't really lying.