If you're a nation which relies on Cavalry and has 2 Military Ideas (e.g. Military Republic, Aristochratic Monarchy, Imperium), a good mid-late game combo is Horse Lords and Cataphracts. This will give your Cavalry +25% Discipline and +100% Offensive. If you've gained the Noble Cavalry / Companion Cavalry "invention", it can make your Cavalry attacks devestating. Throw in some Horse Archers and you've got a fast, efficient and ruthless squadron in your hands. Completely useless during a siege, but keep a stack of Infantry handy to move in and use your cavalry to keep the enemy on the run.
Despotic Monarchy ideas set is good for country development and decent expansion (MCE) with the massive gov bonus of 33% discount on the building contruction cost.
Yeah and if you make sure you've got a high charisma Architect, you'll pay even less. If your King is a "Founder" as well, you'll get huge discounts on construction.