Considering that this is a Sunday afternoon that I'm writing this, I have to say that this song is just about the most appropriate thing for a Sunday afternoon, provided that there is at least a bit of sun out there. And even if there isn't, this could potentially open up the skies all by itself.
What I'm talking about it is this creation by singer-songwriter Miho Moribayashi（もりばやしみほ）and guitarist Kenji Kondo（近藤研二）, aka the eclectic pop duo hi-posi（ハイポジ）. Titled "Kanashii Koto Nanka janai" (Not Really A Sad Thing), this was released as a 1996 single, and it's probably one of the happiest ballads that I've ever heard in Japanese music.
Joining Moribayashi's whispery vocals are an orchestra, a harmonica and what sounds like an old steel pedal guitar to create this adorable melody that feels like listening to mellow pop from the late 1960s or early 1970s. In addition, the lyrics can talk about a romantic relationship up and running joyously on all pistons or a very happy friendship among a bunch of kids. The setting could be a park with the music adorning a hand-in-hand couple or an elementary school baseball team heading home after a satisfying win....and there has to be a dancing segment near the end.