Question

What did king Hrothgar do to the Beowulf?

Answer 1

King Hrothgar presented Beowulf with gifts.

Answer 2

What King Hrothgar actually did to Beowulf after the defeat of the monster Grendel, is that he welcomed him into the Herot Hall and spoke on his honour and celebrated his victory.

Although, it was known that Grendel was a brutal monster to slay down, Beowulf faced it with courage and valour and succeeded in slaying down the monster. This is the main reason why King Hrothgar couldn't "contain his happiness".

After the speech, King Hrothgar symbolically "welcomed Beowulf" as a member of their family. Because, upon defeating the monster, Beowulf now became one of their own both in terms of vision and the nature of the heart.