"I want to make an offering to Ares but I'm at a loss on what to give Him.... Any ideas on what He might like to be offered?"
I get quite a few questions on how to best honour a certain deity or what to offer Them. I have answered this question before, actually, in relation to Athena and Poseidon. As I said then, the major difference between reconstructive religions and modern ones—especially Pagan ones—is the way worship is conducted. Individual worship of Gods as well as patronage is perfectly acceptable in modern religions, but in Recon religions and the ancient Traditions they were based upon, worship tends to be of the pantheon, not so much the one God or Goddess. Worship was generalized in such a way that multiple Gods and Goddesses were worshipped in one rite—and usually in the same way.
The traditional offerings for the ancient Gods were wine, meat and skins from sacrificed animals, and incense. Pretty much anything else is a modern invention; coins, feathers, statues, etc. etc, are all modern. Yes, there were statues in temples, but they were there to house the presence of the God and to give a focal point to worship. Yes, gifts were given to temples, but as payment for the aid of the priests. Basic, Hellenic worship means pouring wine into a fire, means tossing barley groats and incense into the flames, and depending on how hard-core you go at it, to sacrifice animals or give bought meat to the Gods through flames.
Those of us in Hellenismos who came there from the Pagan community often feel the need to give specified gifts; for Ares, I could imagine decorating shrines with weapons and shields, with the colour red (for blood and war but also love), and other symbolic gifts. This is perfectly fine, of course, but please be aware that this is a modern thought pattern. Even ancient shrines were basic places of worship, decorated mostly with practical tools. Temples were places where riches were collected and stored--usually not even displayed prominently.
To answer this question, the best I can come up with is red, grass-fed meat, frankincense incense and some hymns in His honour. I hope that helps.