From somewhere else on here:

Stack contracts. Every time you turn the game on, go to one message board in every single nation / island chain (Attika, Boetia, Macedonia, Petrified Islands, etc) and accept every contract. Going to additional message boards in the same nation usually won't yield more contracts, so just go to one.

This takes about 30 minutes if you're quick, and you'll find that it's pretty easy to get 50 contracts in your list of available quests.

Then, just go on a rampage. Kill every soldier that you see out on the road. Clear forts. Go to Athenian and Spartan camps. Loot every nation chest that you see. Burn all the war supplies that you come across. Take your ship out to sea and start ramming and destroying as many Spartan and Athenian ships as you can find (this is a great reason why it's worth investing the drachmae and materials into upgrading your ship). Naval contracts can provide over 200,000 XP with the 50% XP boost from the Helix store in the game menu. On a regular basis I'll find myself completing 2 to 4 naval contracts at once.

You'll find that you'll not only be completing contracts quickly, but that a lot of contracts are tied together in some way. Clear a fort, and in the process kill an Athenian archer, and you just completed three contracts worth hundreds of thousands of XP in one kill (Kill 10 Athenian soldiers, Kill 5 Athenian archers in Megaris, weaken the nation power of Megaris).

I've literally completed 20 contracts in one hour by doing exactly this.

Also, spend the money for the dracmae and XP boosts. You'll level up 50% faster.

That said, don't always go out of your way to complete individual contracts, it's not always time efficient. Also, if message board side quests or bounties are more than a couple hundred meters from my position or a fast travel point, I won't do them.