Most of the replies have very good answers to deal with unrest, but they need time to work.
If you want a fast solution even in mods like Invictus, Age of Bronze, etc.. use taxes:
Invictus: lax taxes gives you +10,00% pop hapiness, very handy for an expanding empire
Invictus: Decreased Pay gives you +5% happiness for integrated culture. Use it while not at war if you do not have legions yet
Corruption is a real problem for unrest, pay more to get rid of it.
Governor Corruption affects your province loyalty directly, and will cost you money anyway:
Finally, War Exhaustion is great to acquire military experience points but it will affect your POPs happiness as well:
Invoke devokio to reduce War Exhaustion and increase Tyranny to reduce your Aggressive Expansion:
Your rulers are very important too, the game does not tell you but governors finesse and loyalty affect the governor policies and your ruler stats affect greatly your empire:
Look at these governors and the effect of their policies:
And your ruler specially want it with high zeal and low corruption
Also, remember that your officers work for you, sometimes it is better to call a governor with high statemanship to work in your goverment to get the most of its work. Do not use only characters to please scorned families or unexperienced characters, you are losing out greatly!!!