Although the Hellenic Theoi do not REQUIRE us to offer them anything, because of the principle of Kharis, they do appreciate it. This leads us to look at what are good things to offer to Poseidon, both historically and through modern UPG or research.
Some of them are easy, they are the offerings of wine, milk, honey, or even pure water that ALL of the Hellenic Theoi enjoy. The Orphic Hymn to Poseidon specifies that the incense being offered to Poseidon should be Myrrh. And, through cultural continuation, it can be inferred that an offering of flowers or fragrant herbs like basil is made upon setting sail.
Other offerings that would be good, per the book Hellenic Polytheism: Household Worship (put out by the organization LABRYS) would be horse-shaped tokens — either food offerings for burnt offerings or votive offerings, bulls — similarly to the horse ones, and sea anemones. Based on this last, I would also venture that offerings of fish or fish-shaped votives would be acceptable. And, based on pine and wild celery being plants associated with him, I would venture that a fire of pine wood or an offering of celery would be things that also would be appropriate.
Personally, one offering that I have made is of something my daughter made in pre-school. It is of a small jar (probably a baby-food jar) with sand on the bottom and a bluish liquid in it that has small things shaped like fish and other sea life floating in it. It was made by her in class when they were studying about the ocean. After she gave me the jar, I felt that it was a perfect gift for Poseidon. I use it on my shrine to the Theoi as a representation of Poseidon as I do not have any statues to represent any of the Theoi as yet.