I work out, eat healthy, have a girlfriend. What else can I do to increase happiness?

Find Your Purpose. Find your purpose in life. Give your life meaning. This is one of the hardest and sometimes most elusive things to do. Sometimes we go through life doing what we think society wants us to do and achieve things we think we ought to achieve only to find ourselves empty when we reach these goals. Find your passion and purpose.
Find fun things. In addition to everything you do and finding purpose in your life as a colleague wrote, make sure you make time to have fun and enjoy the activities you usually like to do. Also make sure you make connections with friends and people whose interests you share. The more real connections you have to others the happier you will be.