As Scout looked out from the Radley porch, she regretted that she and Jem never gave Boo anything in return for his gifts, but they did give Boo something. What did they give him?

The main thing the children give to Boo Radley is friendship. Boo Radley had been very lonely. He never left his house, and no one ever seemed interested in caring about him except for Dill, Jem, and Scout, who attempt to reach out to him.
Dill is the one who pushes the hardest for getting Boo to come out of his house. Dill understands loneliness, so that is probably why. He appreciates Boo’s situation and does not consider him a monster.

“All right then. What’d you write him?”
Dill said, “We’re askin‘ him real politely to come out sometimes, and tell us what he does in there—we said we wouldn’t hurt him and we’d buy him an ice cream” (Chapter 5).

Boo Radley really responds to these small acts of friendship. He does not mind the children reenacting his story. He seems to find them amusing and appreciates the attention. This is why he begins leaving the Finch children gifts in the tree. In Boo Radley’s lonely life, Scout and Jem are a breath of fresh air.

Summer, and he watched his children’s heart break. Autumn again, and Boo’s children needed him.
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough (Chapter 31).

Boo Radley is childlike in his own way. He relates to the Finch children because they are kind to him. They reach out to him, so he reaches back. They give him the courage to come out of his house, which he had not done since he was a teenager. He gives them the gift of their lives when he saves them from Bob Ewell, and they give him the gift of friendship.