This is a very thought provoking thread.
Is content not the same as happy? Happy and sad are two extremes, but when you find peace with yourself and surroundings, that will lead to contentment which is the middle ground.
If you were to experience great happiness with your significant other, would that not mean that you would also be experiencing great upset or sadness, whereby you compare the two extremes to come up with the definition of 'happiness'?
But to answer your question, I do things for my significant other that I know will make him happy. (Back rub, cuddle, ask him about his interests, serve his favorite food) When I feel his joy, then I am happy, and he does the same. We are happy, but also content.
Just my thoughts on the concept of contentment, or am I missing something else that you are trying to convey?