My dad should be coming back in a couple of days. Is there anything I should buy him to congratulate him. Going to get some mitai.

JazakALLAH-heir.

Mabrook to your dad! May Allah accept his Hajj and bring him back safely. Ameen.

As such, there isn't anything stipulated anywhere that you should buy.

I was going to suggest a new Qur'an and new musalla, but I guess he might be bringing some with him!

Is there anything you can do in the house that your dad might be pleasantly surprised to see?

The best thing, other than a physical present, is from now onwards to be the best son that you could ever be. For the rest of you father's life (and mother's, if she is alive) to do things without them having to ask you, to ask if there's anything that needs doing that you can do, to always obey them, to not show annoyance if they ever rebuke you, and have a special meek tone of voice reserved especially for them. And of course to pray for them.

Other than that, up to you really!

May Allah take us all for an accepted hajj. Ameen.