Aria The Origination, K-On! Recommendations
Both K-On and Aria are strong in conveying a relaxing atmosphere through pleasing visuals and a very beautiful soundtrack.
While the shows might seem rather different on first sight, the concepts are largely the same. You get to watch a small group of friends grow subtly, but steadily, going on little escapades or just acting in their respective environment.
Characters are strong in very similar ways (including all stereotypes and relationships), the melancholic and slightly nostalgic development and conclusion invoke very similar feelings.
Moreover I would recommend watching Aria first, since it is (imo) more successful in overcoming stereotypes and offering a broader experience, while still accenting