I got Michelin Energy Saver 185/65/15 on my City 2011 and the mileage improved from 13.5 to 14.5, the tires are really soft and grip real good.
The AC filter was purchased from Honda for around 1200, it does stop some dust from blowing in but to be honest i don't really feel any difference.
Changed the horn, cost 1000. Now people can actually hear it.
My advise on the lights. Say no to HIDs. They will never fit well in the reflective housing of the City. Change the stock bulbs from Osram Bilux to Philips Ultravision. Cost 3000. They will improve visibility nearly 150% and you can high-beam away anyone with HIDs off the road