Based on the search results, there are a few different things you can offer to Selune in Baldurs Gate 3, depending on the context:

• Stormshore Tabernacle: You can offer 10k gold to the statue of Selune at the Stormshore Tabernacle in the lower city, and you will receive a daily blessing from the statue.

• Owlbear Cave: You can offer a Selûnite Prayer Sheet to the Seal of Selune to unlock the Gilded Chest of Selune in the Owlbear Cave. The chest contains various items, including a Silver Necklace, Blood Stone, and an Idol of Selune.

• Other contexts: There doesnt seem to be any specific item to offer to Selune in other contexts, such as at other shrines or during quests. However, its possible that offering gold or other valuable items could have some effect, as some players have reported that the amount you offer affects the quality of the blessing you receive.