i was really hoping we wouldn't go against Sparta but okay. First off most of our troops are peltasts and ranged troops far faster than the spartan phalanx even though less organised. Hopefully we have gained some archers as well men which the spartans will be largely bereft of (if i remember correctly helots were their auxillury troops). Thus i would suggest trying to pest the allies and hopefully seperate them into pieces as well as simply crushing them with our superior cavalry before the spartans arive.

Up North i think the men need to hold our ground and try to win over any thracian or getian tribes we can. it would help if the enemy was divided. That and any additional allies would be nice. (perhaps try any of the northern steppe tribes as well. Holding the stepes (or being allied with their leaders) would also be useful and i think something small as trading goods and promises of freedom and maybe a few tracts of land would bring these valuable warriors to our side and tie up more of the other princes troops.
3. improving the fortress of athens may a wise choice it also may question the amount of time the persians will stay. I fear that the athenians will be nervous if it seems the persians are planning a long term stay so it must be clear that we do this for them and for greece not to build ourselves a fortress