An hour a day all summer is enough time to develop an app. However I am not sure this qualifies as "productive" or "money-making!"
In seriousness, it depends a lot on where you are in your life, and what your goals are. I would give very different advice to a student on summer break than I would to someone in the working world and with a family.
You mentioned that your summer will be very busy, so how about using that one hour a day to just relax?! :)
If you're busy all summer, relax for that one hour a day. :) Take a walk, go for a cycle, have fun!
Learn something new. Spend one hour a day messing with code. Or watching up to 2 or 3 video tutorials. Or reading one hour of a programming book a day.
You most definitely have enough time to develop an app
Borrowing from Richard Hamming via Paul Graham--
* What are the most important problems in your field?
* Are you working on one of them?
* Why not?