You can just turn the pinging off. Then they're going to go to sleep by themselves. I've got a hobby app and it goes it sleep. The dyno comes back up in seconds. It's not bad.
Google App Engine also seems like a good place for a hobby app. More constrained, but Heroku is not really that much more flexible compared to totally free-form architecting.
Actually, looks like you could leave pinging on and Heroku will just force "exhausted" free dynos to sleep after 18 hours:
"During the beta period you’ll only get an informational email. In the future, we will force exhausted apps to sleep until they have been idle for at least six hours of the past twenty-four hours." -https://devcenter.heroku.com/articles/dyno-types#dyno-sleepi...
Google App Engine also seems like a good place for a hobby app. More constrained, but Heroku is not really that much more flexible compared to totally free-form architecting.