I think spies are by far the easiest to level up.

Plus the AI seems lax on repairs, so instead of breaking a ceasfire with a nation, send 2-3 spies and an assasin or 2 and blow up churches and law offices and wait a few turns, chances are you'll get a rebellion and an ousted nation.

I havent tried this much cuz it seems like cheating but wonder if the AI is more agressive to CI and repairs.

On a side note, I would rather of had an option to build a special inn or something that provides a bonus to counterspying so you dont have to tie up so many agents.

I have about 15 spies on my payroll that just sit there.