Journey! Very chill, great for meditation, very beautiful. I wouldn't call it stereotypically upbeat and positive, but it's definitely not dark and gritty. It's very sublime.
Okami, but that has been said.
Katamari is a great cathartic silly game.
Games like Mario Kart or whatever are nice upbeat fun.
If you want something with a happy plot and characters and stuff, how about Ar Tonelico? It's so happy I get diabetes from the sweetness. ^^; (Although you'll have to deal with its framework of immature humor, but just avoid 3 and you're fine.)
The Touhou Project is fun, challenging, and sugary-sweet also.
