I've never played at anything other than normal. I understand from other posts, that the more aggressive difficulty levels actually make it easy for the human, since the ai beats itself senseless in all wars, making ai countries relatively weaker. I haven't heard anything about the weaker aggressive difficulty levels.
For something different (if you haven't already tried), try using the other economic options in the IGC. Chaos Hurts, for example, severely restricts income when you are at lower stability - it's a challange (for a while). When it takes 8 to 15 years to recover from a religion change, you really appreciate having money (and high stability). If you are really interested, check out the IGC+ threads in the Scenario forum - the objectives of several people (I am one), is to reduce the amount of money in the game by a very large amount.
Another alternative is "role playing". Adopt various rules to govern your own behavior. For example, don't exploit various ai weaknesses, only do some aggressive action if there is a good, "real life" rational for it, etc. If you are in to England, it's huge fun to do all the religion changes that actually happened (especially combined with restrictive economic settings, like Chaos Hurts). Play Austria as "defender of the faith" and so on.