Answered by Sayyidi al-Ḥabīb `Umar bin Hafīẓ (may Allāh preserve him and benefit us by him).
What is the most beautiful gift that someone can give to the Messenger of Allah ﷺ in the month of Rabi` al-Awwal?
In reality we are unable to benefit him in any way but what pleases him is for us to understand his words and his teachings and then by acting upon that to attain high stations. He is content with that which benefits us.
One of the greatest of those things (as we understand from the Book and the Sunnah) is for us to be a means of guidance for someone. That person may be a non-Muslim who accepts Islam or a disobedient or heedless Muslim who repents and becomes upright.
Another thing is giving life to his Sunnah and establishing it. If we establish the Sunnah in our homes and teach it to our children this makes him happy because it is us that benefit from this. He said in the hadith:
“Whoever brings life to my Sunnah during the time of corruption of my Ummah will have the reward of a martyr” (or in one narration: “one hundred martyrs”).
Another thing is to do good to the creation in general, to the believers specifically, and the elite of the believers more specifically.
A further thing is to strengthen our love for the believers, for the people of the Prophetic Household and for the Companions.
Finally, we should make big intentions in Rabi’ al-Awwal, such as gathering together to celebrate the birth of the Prophet and spreading knowledge.