I'm partial to instrumental and/or electronic. Some of my favorites:

In Love with a Ghost

Potsu

Snail's House (Before the Last Summer Ends)

DJ Okawari

Kirinji

Forrest.

Guitar

Loop Pool

Probably some one-off songs I'm forgetting but oh well.

Bakermat, Ginger Root, and Lucky Kilimanjaro are also very good, but they have a bit more energy to them so not sure if they count as relaxing lol