Love your post, Liz33, and I'm glad to hear that you've found something so helpful in bicycling!
Not sure if it's up your alley or not, but I took this MOOC (free, online course) and liked it a lot:
https://www.edx.org/course/science-h...eleyx-gg101x-3
Anyhoo, riding a bike always gives me a happiness boost too, and I'm not sure why. Maybe it's because any sort of exercise tends to create positive emotions. Maybe it's the wind on my face and a "connection" to the environment and community that comes with cruising along in the open air like that.
Maybe it's because I used to ride my bike all the time when I was young, and riding now as an adult sort of sends me back in time, turning me into a giddy kid again. Freedom meets play. Tough to beat that combination!