In terms of games that fit "relaxing and upbeat" with sidequests and exploration... I would recommend the following:
I don't know what you already play but I love these...
-Stardew Valley: mix of old Zelda games, older harvest moon games, and animal crossing. It has an insane amount of content and choice as to how you play. You can explore mines/dungeons and enjoy some combat, fish, farm, develop relationships (all the way to marriage if you so wish), and shape your farm however you would like. It's criminally cheap for the quality you are getting. The best part of the game is discovery, there are so many surprises you can stumble upon in your gameplay.
-Fire Emblem 3 Houses: another great game with a huge amount of content without being overwhelming. Play how you want. There is a lot of replay value and hundreds of hours of fun to be had.
-Littlewood: a more simplistic but enjoyable and relaxing game. Very upbeat. Currently on sale.
-Dragon quest builders 2: tons of content, very relaxing and mostly upbeat. There is a "jumbo demo" available for free on the shop if you would like to give it a go, if you do so, keep in mind that the first bit on the ship is a tutorial more than anything and not indicative of the rest of the game.
-Paper Mario and the origami king: Agreeing with the person above, very upbeat and reasonably fun.
-Pretty much any pokemon game: I am not a fan of the series myself but it is certainly relaxing and upbeat.
-Mario+Rabbids Kingdom battle: Hillarious, relaxing and upbeat! Love this game, the graphics are gorgeous too. It is often on sale and worth the price.