This always sounded like fun to me. I wouldn't expect to get rich but seems like a nice reason to get outside and enjoy nature like looking for nice shells on the beach.
I thought about that yes, but I dismissed under the assumption that to even reach 90€ of profits in 8 months I would have to reach thousands of users. But then I would have to provide some kind of support for more users, the UX of my app would be crappier because of ads, and I would have worse reviews (currently 4.6 out of 5). More work for the same result.. :/
I have only one and that is my website where I teach JavaScript + ReactJs. I cannot say that it is really profitable, but at least I get a bit out of selling an eBook as pay what you want. But it's nothing compared to the time I invest in it.
https://logmyhours.com - Time tracking and invoicing. My only side project ATM. Doubled the revenue from Jan already. Hoping to go full time next year!
Did a bit of Bing Ads last year (Had much better results than Adwords and Facebook Ads). Nothing really now, mostly just working away at new features and making improvements.