Gentle, atmospheric piano and ambient textures. Kimi no Omoi, Sono Negai is a Windows visual novel from 2003, and its complete soundtrack sounds like staring out a window on a rainy afternoon. "Promenade of Sunbeams Streaming Through Leaves" sets the tone with soft filtered light in audio form.
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"Desert of Water" and "Traveler in Fog" push into more melancholy territory, almost uncomfortably still. "A Thought Like Powdery Snow" and "As With Fallen Leaves, Dancing and Scattering" have those long poetic names that early 2000s Japanese PC games loved.
The arrange versions of a couple tracks round things out without reinventing them. Grab this one if you need background music for staring at the ceiling and thinking about someone.