AI is just not attacking enough....

[Tomgs]

Another thing don't use any bonus points in happiness at the start of the game, maybe take a few point off of it to make your planets tend to get mad without help.

Also your happiness type has a lot to do with it. If you are bloodthirsty colonize a lot, don't build many ships, make treaties with the AI, and lose a few battles. If you are peaceful declare a war or get some non intercourse treaties. If you are neutral do anything. They don't like changes so anything you do will make them unhappy, go explore and meet new empires.

[Mithra]

Another idea to make the game a little harder is to start a game with a lot of empires and when you are getting to strong to beat them let some of the computer opponents surrender to each other. That way you get some strong opponents instead of a lot of weaker ones. Also you could also support them with some lacking tech or planets. By letting them trade with each other or your own empire.