Here are offerings to the gods without looking like offerings to the gods
Books: Great for Athena, but can be offered to other gods as well. I offered “The Art of War” to Ares once.
Drawings: Of the gods or of anything else. If drawn for them they work
Poems/Written works: Poems, stories, songs. They all can be written or typed and printed and set on the alter or other area dedicated to the gods
Jewelry: Any kind. They just look like you set jewelry down
Keepsake Box: I did this as a offering box and also as an offering to Artemis when I was younger
Coins: Good for Charon, Hermes, and Hades
Flowers: Everyone loves flowers and they just look like decor
Statues/Figurines: It’s art. As long as you aren’t overly decorating them or putting other offerings at their feet no one will think “Hey now that there’s an offering”
Stuffed animals: My kids do this. Recently a stuffed owl was offered to Athena
Sea Shells: You can just put them in a pretty box or jar.