The fun quotient depends upon the persons involved and their interactions with each other. There are no medicines or supplements that will replace communication and love. If you want to make sex more fun, find out what your partner likes and avoid things that he or she dislikes. Spending time together inside and outside bed is also very important.
Think of romantic situations from your own life or from movies and try and recreate those. Try and find out ways of pleasing each other, praising each other.  
There is fun when there is basic tuning between two people. And that does not come from any tablets or medicines. It needs two live, active, interested people.