What if every fight with your significant other could be resolved, or at the very least stabilised, by a happy mediator? If that sounds too good to be true, don't underestimate the power of a mason jar. Think of it as an in-case-of-emergency project to bust out in times of relationship distress. Whenever you and your partner have something sweet to say about one another, a fond memory you shared together, or a silly moment you want to remember, write it down on a piece of paper and put it in the jar. Don't open any of them until necessary.
So if you two get into a fight or are going through a rough patch, you have a bank of reminders to lean on. Take turns reading each note aloud, and chances are that whatever problem you're going through won't seem as much of a problem anymore. It won't make your issues go away, but it will emphasize what's most important in your relationship, which is your love for each other.
There are so many ways you can apply the happiness jar to your relationship. Make it your annual tradition together and read your notes at the end of the year or on each anniversary, use it as a way to survive long distance, create individual ones to gift to each other, etc. Get creative!