There are no things that "make you happy whenever you do them, and that you never get tired of". Even considering things that you like to do often, different of them will work for different days/times and different circumstances.
Therefore the list of things to do every day that will make you happy can't be fixed. Instead, I propose making (entering into the app) a long list of activities that make you happy, which you would update whenever you want and browse in the morning, or in the evening (preparing for the start of the new day). Every day you'd pick up a few things from the list to do on that day (whatever you feel like doing today/tomorrow the most). You could start with 3 different things to do each day.
You'd do each of those things at least once on that day, but preferably more. That would be your determination and the "deal" with the app. Once you did a certain activity chosen for that day once, you'd indicate this in the app by simply opening it and tapping on the activity icon displayed on the screen, the app then would register it as "completed" and you'd get reimbursing dopamine hit.
The user could choose how many activities to do on a certain day and how many times each of those has to be done. If you chose to, for example, give three free hugs that day, you'd tap on the "hugs" icon in the app after each hug, after three taps it would be marked as "completed".
How good the day was will depend on many factors and the app should aim to figure out the best activities that contribute to your happiness and how often (and if possible when) you should engage in them. Therefore at the end of the day, I suggest rating not the general happiness level of that day, but how much those activities contributed to you feeling better on that day, you might as well rate each activity separately.
The app would slowly learn which activities contribute the most to you feeling good and would suggest those as tasks to complete for the next day (the part which was initially manual would be replaced by recommendations).
The very important part is for the app to try figuring out chronological patterns in your preferences for and positive ratings of certain activities - maybe you chose running on Tuesday and liked it, then didn't like it on Friday, but liked it again after a week, why is that? Is there a pattern to this that can be guessed by the algorithm to offer you the right activities at the right times? That should be the core focus of such an app, in my opinion.
For the sake of variety, the app could also suggest new activities you haven't tried yet based on your past likes.